This is a common tactic employed by unscrupulous slumlords. Unfortunately, you already paid--that constitutes admission that the debt was valid. There's nothing you can do now. If you'd asked this question before you paid, you could have armed yourself with the information needed to handle this situation without paying. You had video evidence of the final walkthrough demonstrating that they were happy with the condition of the unit, fer chrissakes. Showing that to them would have gotten them to back down, easily.

Anyway, that's water under the bridge. But for your future reference, and for other redditors who may have a similar question: anytime a shady landlord withholds your deposit or claims unfounded damages, what you should do is request an itemized list of said damages. If they don't have one, then the buck stops there. If they trump something up, photos and/or video will save your ass every time.

A few years back, I had a landlord try the same shit on me after I moved out. She emailed me claiming vague, unspecified damages and hoped I would understand why she had to withhold my deposit. I ignored the email and waited until 2 weeks before the deadline for deposit returns in my jurisdiction (1 month, YMMV). I sent her a certified letter demanding my deposit in full. I said I would take her to small claims court if I didn't get the whole thing back (or a partial refund + itemized list of damages). I also reminded her that, if I won the case, treble damages were awarded to tenants whose rights were violated by landlords in our jurisdiction.

I got a check in the mail the day before I would have had grounds to file.