I wouldn't be too concerned given the generality of their statements. You have the video footage form day 1 indicating that there weren't any significant mark ups, dents, etc. in the walls. This is also time stamped. You have their security deposit; when they leave, use that security deposit to fix up the "lots of" marks, dents, holes, etc. that they are attempting to protect themselves from having to fix (assuming no other damages). If they believe this to be unfair 12 months from now, let them sue you. They won't win for a number of reasons, but here are the two main ones:
1) "Lots of" is a non descriptive phrase that virtually means nothing. They should have taken digital pictures of their own documenting each of these blemishes to protect themselves. The fact that they won't be able to provide them (and if they do provide pictures you have your time-stamped footage validating how it actually looked during the walk-through) will be enough to sway the judge to see the merit of your defense. Remember, in the US the burden of proof is on the plaintiff!
2) The time/costs of suing you for the damages of fixing up these holes, dents, etc. is immaterial and would be inadvisable for them to pursue. Simply, the cost/benefit isn't there for them.
I wouldn't address this as a concern right now because it will damage your immediate relationship with the tenant and remember that you still have to deal with them for the duration of the lease! You've protected yourself with the video footage; I don't see any reason to instigate a fight here when there is no merit to their claims to begin with and you have time-stamped evidence to prove that. This is a fight you can win at the end of the lease AFTER you've collected your rent. Right now, it's a losing fight that only creates problems and a strained relationship, but based on how your tenant is acting, I suspect this isn't the last problem you're going to have with them.
While I don't speak from personal experience here, my father has dealt with tenants like this in the past and this is how he has successfully handled it.