The gemarah asks that teharos are written, so why does the Mishna say that they are just "have what to rely on" It answers that the Mishna is referring to the drasha that Mikva needs 40 seah and it quotes a braisa that darshens from the pasuk that a Mikvah a) can't be mayim sheuvin and b) must be forty seah. The question is why doesn't the gemarah say that the Mishna is referring to the first drash of sheuvin instead (or as well as) the drasha of 40 seah?
The Turei Even asks that why are we calling these drashos "have what to rely on". If these drashos are only considered a semach, then every drasha can be called just a semach? He answers that the drasha of Mikva tells us that every person must use the same size Mikvah- even a midget-, even though the pasuk is only mashma that a mikvah must be large enough for the person going in it. Similarly by sheretz, every shertz uses the size of a infant snail, even though the other minim of sheretz are larger in infancy.
The orignal question is answered. The mishna is not calling the drasha that you can't use sheuvin a "semach", it is a 100% drasha, like it is written bfeirush. It is only calling the second drasha a "semach".