I think what he was trying to describe is when you are expecting the enemy team to start switching in Fakeout-mons as a response, you should release a dyna-psychic attack to set up a field (though you still can't stop Roar that way but at least you have a better chance in taking the roarer out before then).How? You have to actually be able to use a move in order to do anything, and the point here is that it's relatively easy to deny a Dynamax Pokemon the opportunity to ever take an action with Fake Out/Roar. As far as I know, there is nothing in the game apart from a couple of obscure abilities that confers immunity to phasing moves, other than if the trainer has no other Pokemon.
When a mechanic is showcased, it's good game design to have counterplay to it (if it's strong enough to necessitate it), but not so great for there to be a single commonly-learned move which hard counters every Dynamax Pokemon. If the main counterplay involves preventing the opponent from using the shiny new mechanic at all (or limiting it to one turn), that isn't fun to play and isn't much fun to watch. It doesn't help that phasing moves feel like an oversight that's being exploited to completely trivialise the mechanic, rather than being intended to be a core part of gameplay.
That said, it's entirely possible that with the mechanic so far not being particularly overpowered, people just won't see the need to bring phasing moves to deal with it. Dynamax has inbuilt counterplay in that the most useful parts of it so far seem to be setting field effects, which is a problem that naturally cancels itself out with how easy those field effects are to clear or overwrite. And the stat boosts we've seen so far are fairly tame. If that's the case, it probably won't make much difference whether Dynamax can be cleared with phasing moves or not.
Probably this time GF is trying to avoid the same situation as Megas (where they will remain powerful forever until they die) or Z-moves (one-turn mega nuke that is damning hard to block). Everybody can now Dynamax (assuming no other limitations) and be a big boi so you don't need to strictly build a team centered around a pokemon with the move/forme and rely on it to breakthrough, but with such incredible versatility they probably thought make it generally weaker and more prone to force switches/stuff in general is a good limitation.
I'm still wondering how does the dynamax gauge work at this point. Do you need to, idk, take out a few Pokemon or have some of your Pokemon taken out to charge the gauge? Or there's some other use/conditions for it?