It seems to me like all the angles of this classic argument are true at the same time.
For instance I recently found a mentorship lead. Even though my default plan is to assume that there will be zero mentorship, at the same time my stance is to jump on any mentorship lead that seems good and has the terms I want, since it can potentially take months or years off the learning curve.
Zero Sum Angle
I'm definitely not going to share who this potential mentor is right now, because it could threaten my ability to get started with and establish a relationship with him. He could already be almost fully capped for the number of students he will work with (he does full-on 24/7 availability call/e-mail mentoring). If I start shouting his name from the rooftops before even getting started, I may get "cut out of the deal" even if it's only temporarily, but I have no logical reason to wait on that.
Unlike 95-99% of people who get started in this business, I don't just want to get started. I NEED to. My entire existence right now revolves around this. Therefore I'm not going to let the flower child approach ruin my chances, especially when most of my competition probably is already living a cushy life compared to me. You think I'm going to just hand a big cleaver to some random middle/upper-middle class hapless student who's been "kind of thinking about" real estate for the past 4 years so they can chop down my poverty level 3-month long journey into this? HELL NO.
This is because getting started with this guy and not getting started with this guy in the next couple months could mean the difference between going from 1 step above homeless poverty to living in a house on my own, or not, in less than 1-2 years. Missing this chance may add years of poverty to my timetable. I'm not going to sacrifice the opportunity to "be nice" especially to some random less desperate and less decisive newbie. Dirty game yall.
Abundance Angle
However, if I secure a mentoring relationship with this guy, and then actually close some deals and get some thousands coming in, I will no longer be secretive about who he is. I may not be shouting it from the rooftops, but usually when you do that, nobody believes you anyway ;)
I would be open to sharing then, because the threat of being cut out is no longer there. Also, the more I learn about this real estate business, the more I see that there are huge amounts of opportunities all over the place and for different angles. The chance of any new students of the same mentor cutting me out of any specific deal are super super slim, and even if they did cut me out of 1 deal, there's an even lower chance they would end up competing directly with me for the exact same next deal, unless they were local to me.
If they were local to me, that would be an opportunity to ally with them in some way. It might water down the profit per deal, but it could also secure bigger deal throughput and easier deals. However, if it became apparent that this person did not intend to share any of the chips, then I would go back into full ruthless zero-sum mode and up my game until I can grab all the chips as fast as possible, and more for good measure (having someone make a grab for all my chips does wonderful things to my brain). However, even in that situation, chances are there will still be opportunities that do not overlap, and more opportunities farther away. Therefore being locked fully into competition is not necessary.
It seems that in the process of forming relationships with others, tons of leads are generated for all kinds of avenues. For instance on BiggerPockets I already scored twice, not on real estate leads, but on small cash leads. I'm building a list of those who showed me favor above and beyond a random forum post, for later profitable recompense.
In addition, if people are successful because of my referral to this guy, they will be grateful to me and possibly lead to some very nice profits down the road directly or indirectly from those relationships.
Conclusion
Abundance tactics work wonders in the long term, although in some situations they can seriously derail things. Such derailments aren't really an issue to someone who is already doing good, but could feel like life or death to someone who is poor, or just driven to be hyper-competitive. Someone that's doing well won't care about losing individual deals, they are focused on how well their overall operation is serving them, and couldn't care less about the details.
Zero sum tactics are both useful and a valid way of looking at things in certain cases, but the overall meta-game is so large that they fall apart when applied to everything, especially in the long term, unless of course you're so greedy that you want it all, in which case have fun with that :)