This part is quite true, he was working on Bitcoin Core, then switched to XT when his pull request was rejected:
https://github.com/bitcoin/bitcoin/pull/6341
History of Gavin's Pull Requests for Bitcoin Core:
https://github.com/bitcoinxt/bitcoinxt/pulls?q=author%3Agavinandresen
You can see how the Bitcoin Core > Bitcoin XT > Bitcoin Core chronological fits quite nicely.
It also matches up very nicely with the aforementioned mailing list posts.
I want to make it clear that I'm not suggesting that Gavin is a *bad actor*, I'm just supplying evidence to support my claim that Bitcoin XT is considered a laughing stock from the Bitcoin Technical Community.
I do think that it is fair to suggest that Gavin was interested in Bitcoin XT more than Bitcoin Core for a period of time as Bitcoin XT accepted his BIP 101 pull requests.