He says you can use multiple DEs, but doesn’t say you can’t switch for GTK to QT or the other way around.
This is not a Manjaro video, from what I can see, so the Manjaro team does not have any say in what is in it.
Moreover, Qt and GTK are 2 completely different things and you can’t switch between them, so he is correct in that.
About multiple DEs, yes you can use more than one. But, just because you could, doesn’t mean you should.
I think you already posted about that video earlier - did you forget that?
And the video is from one out of 100’s YouTubers with an opinion.
You switch around and mix as you see fit.
Be aware that Plasma and Gnome may have conflicting services - most prominently Gnome keyring and KDE Wallet.
You may also face strange issues with incompatible theming.
Other strange things may happen if mix Gnome desktop and Plasma desktop but Qt and GTK as toolkit does not conflict but getting a consistent theming is difficult.
Duplicate of This video says you can just switch your DE?