technically, it's possible to download the file and play it in mplayer. i have never been able to use mplayer for .flv files without it closing with an annoying error every time it plays a video. the video plays first, so it's not useless, but it's always given me the impression that mplayer is a piece of... i like vlc, which has the same problem with some videos (but not .flv.) but i haven't got vlc installed and the main apps tc lacks imo are pcmanfm and qemu. i wish i knew why other people enjoy mplayer more.
if rms is like the guy in the video you posted, then he won't even install flash. i'm unhappy that gnash is not available to play youtube videos, and if it was i wouldn't use adobe's. so long as youtube is functional, that's enough in terms of flash support. it's too tedious to browse youtube videos by downloading them all the time, even if you watch them while you download.
according to stallman that makes me a "coward," but he's right about most things. we all say dumb stuff at times, he's just less afraid of doing so than most people. the list of things i don't do because i care about the same things he cares about would make most people think i'm nuts, but giving people a little leeway lets more people fight for software freedom... even the ones that get it but aren't all-or-nothing. (in other words, most people.)
i never buy "you're either with us or against us" from anyone, free software is based on the freedom of speech, and free speech is based on the right to think for yourself- even the right to make mistakes and be imperfect. (along the theme of the video and your question, adobe flash is imperfect in more ways than one.)