guaranteed. only a tiny microscopic portion of the plant needed to hitch on a snail shell and then vector into the tank. its unpreventable in my opinion, only what we can do after the import matters, and to me its whatever the most creative and thorough removal by hand would be.
I know the strictest QT could allow these riders to fruit out and be identified before DT transfer, but so few people will do that hence all the follow up wars we wage on bryopsis lol.