I was able to set up the Yeelight with Google Assistant on my phone. In my case, when I clicked to link the Yeelight (as I described in OP), I was getting a differently localized landing page for Yeelight. Nonetheless, after I set Chrome to be my default browser, it worked fine.
I also had IFTTT, and yes it works great! I'm not sure how nice Google Assistant works in languages other than English - if you can set up custom voice commands like you can in IFTTT -, and I'm not sure if using one or the other makes a difference when using LAN control (to avoid lag).
FWIW, I'm actually using both now: I have the default commands set up on Assistant (turn off the lights), but also made some custom IFTTT commands (dim the lights; give me more light; turn up the lights; I'm going to bed, etc) with some nice Assistant replies (e.g. IF I say "I'm going to bed", THEN lights turn off and Assistant replies "nighty night!").