Unless the tenants did something to cause the blockage or did not notify you when they saw the flood, they aren't liable for it. 

Tenant also agrees to the following:-All drains, waste pipes, and plumbing are accepted as clear by Tenant at the time of occupancy, and any material blocking them after occupancy shall be repaired at Landlord’s cost, unless determined by repair person that such blockage occurred during the time Tenant controlled the Premises and due to some fault or negligence of Tenant, specifically excluding blockages caused by roots or backups from the street.

Sometimes you can tell because of rust staining and buildup on the outside of such affected pipes,   but even without that they can be weakened considerably from their original state.On the drain side galvanized branch drains  (1.25 through 2" galvanized pipes)  can also rust out or clog up as the rust on the inside is very rough and catches hair, grease,  and other debris and will eventually need aggressive rootering,   which if the pipes have rusted enough may open up holes in them.I should note CAST IRON (hub and bell) old sewer pipe,  at least the 3" and 4" stuff seems to have a bit longer lifespan than galvanized steel;  it still rusts but doesn't crumble and flake and encourage blockages like steel does,   so an old house with a cast 4" sewer stack you may be fine for many decades more if you just replace all the branch lines going into it but leave the main stack.   

Building in a downtown area typically involves privacy fencing the entire site, permits for road blockages, fights with neighbors over port-a-potties, and difficulty with noise ordinances. 

They disclosed that there was a blockage in the drain and hired a plumber who used a water jet to clear what they claimed were flushable wipes causing the issue.

These could lead to leaks or blockages, requiring costly repairs or replacements down the line.

If these repairs are left to the tenants and they fail to carry them out, it could potentially lead to a decrease in your property’s value.Here are a few:Structural Repairs: This includes repairs to the walls, roof, and foundation of the property.Plumbing: Any issues with the property’s plumbing system, such as leaks or blockages, are usually the landlord’s responsibility.Electrical Systems: This includes wiring, circuit breakers, and outlets.

Pay it but if it happens again it's on then because you just cleared their line...unless its tree roots or the main line has a rare blockage. 

If no GD, any blockage service call will certainly be tenant responsibility to pay.

     - Check for blockages and remove debris.