They'll be usable. (And it would be incredibly bad for them if they weren't, they would basically be burning money by printing cards that are worthless outside one event, which would also make people who are entering in part to get started with the Pokemon TCG very, very angry.) They probably just won't be the big chase cards. (And really, a card like Sycamore is rather unbalanced for a Sneak.)
The one curveball I could potentially see them throwing is making the cards in these packs of their original sets, and not a new set. This would allow them to, say, reprint cards from Expanded without allowing them into Standard. I don't know how difficult from a printing standpoint this is, but I guess they do manage it with the Battle Arena decks, so it's probably possible.
Either way, this change is probably for three reasons. To make it less likely someone loses due to not pulling any useful Trainer Cards, potentially making Sneaks more competitive and fair, to give newer players a start on their cardpool, and, if my speculation on keeping the set symbols is wrong (which it likely is), so they don't have to keep finding space to reprint things like Potion and Switch every few sets. They can just put the cards that keep showing up in Japanese decks in these instead of allocating mainset space.