Where have you heard this rumor ??
If Nintendo is set to 'do in' DSTT & close their site, they've also got to go for all the others [Acekard,CycloDS,DS Xtreme,EDGE,EZ-Flash,R4 DS,Supercard etc] @ the same time - otherwise the users of flashcards will just switch to another.
They will also have to also close down all the sites that sell them, & I bet that even if they DO manage this amazing feat (afterall no-one has managed to shut down all the Pirate download sites) - another manufacturer will crop up with a new flashcard.. & so the cycle will repeat.
This will be an impossible thing for Nintendo to achieve.
They may think 'We'll sue/shut down one flashcard manufacturer & all the others will disappear' but somehow I don't think so.