Talk didn't have its own client outside of the Gmail panel when it launched. It would have been much harder for Google to bootstrap at the time, if they hadn't been able to just point people to existing clients that supported XMPP. Third party clients made Talk useful faster (thus popularizing it). Sure, several of the third party clients were pretty good at reverse engineering weird protocols while also supporting open ones, but there's no guarantee they would have bothered with Talk, especially if Talk existed only in a web browser.

Interesting, I seem to recall an entirely different order of release. Blame it on personal bias (was already using XMPP at the time of Talk's release) and/or aging memory, I suppose.

I remembered it exactly the other way round and also wasn't sure. It's curious/disturbing that events barely over a decade past are difficult to google. 'What was the first gtalk client'. 'When was gtalk integrated into gmail'. 'When did gtalk get an OS X client'.