Considering past events such as this.[1] I think it is more likely that the F.B.I are aware of TAILS and its limitations against certain adversaries and choose not to comment.
Every single major intelligence organization will send people to Def Con. They'd probably won't advertise who they work for, but most years there's a "spot the fed" contest where if someone is informally confirmed by those near them, they get a free t-shirt. I'd assume every agency knows every mainstream tool. I find the contrast between an NSA presentation at Def Con 26 and the CIA not having any documents about Gentoo Linux pretty funny. It's surprising to me because ChromeOS is built on Gentoo.
[1]: https://gizmodo.com/report-facebook-helped-the-fbi-exploit-v...