Eh, I'm skeptical that a chat customer-service flunky really has deep insight into high level regulatory-compliance decisions -- especially surrounding regulations that have only been floated as a proposal and which don't, yet, even exist (edit: the guidance seems to have moved forward, but only after amendments making it clear that the FCC specifically wasn't banning open source firmware)

TPLink has always been a shit company when it came to firmware, and routers with locked firmware predate by decades this FCC proposal. I'm not convinced there's a cause-and-effect relationship here.

Edit: Anyway, from the horse's mouth back in Nov 2015 after the comment period closed on the proposal:

"One immediate outcome of this ongoing dialogue is a step we’ve taken to clarify our guidance on rules the Commission adopted last year in the U-NII proceeding. Our original lab guidance document released pursuant to that Order asked manufacturers to explain “how [its] device is protected from ‘flashing’ and the installation of third-party firmware such as DD-WRT”. This particular question prompted a fair bit of confusion – were we mandating wholesale blocking of Open Source firmware modifications?

We were not, but we agree that the guidance we provide to manufacturers must be crystal-clear to avoid confusion. So, today we released a revision to that guidance to clarify that our instructions were narrowly-focused on modifications that would take a device out of compliance. The revised guidance now more accurately reflects our intent in both the U-NII rules as well as our current rulemaking, and we hope it serves as a guidepost for the rules as we move from proposal to adoption."

If TPLink really is locking out firmware "because of the FCC", they've simply misinterpreted things.