The gemara says that the beis has'tarim, hidden areas of a woman's body but still external, such as the inside of the mouth and nose, don't need actual contact with water but must be cleaned from chatzitza so it is fit for contact with water. Tosafos 87a (d.h. pascha) writes that we tell a woman to open her mouth so that it is fit for water, just as we are makpid for chatzitza inside the mouth. The simple reading of Tosafos implies that if a woman keeps her mouth closed while toveling, the inside of her mouth is not "fit" for bi'as mayim and it will invalidate the tevila (but it is possible that the real concern of Tosafos is the chatzitza created by sealed lips). Tosasfos (d.h. kol) clearly holds that the requirement of ראוי לביאת מים for beis hastarim is d'oraysa, because Tosafos learns this from pesukim. However, the Ritva in Kiddushin 25a writes that the requirement for the beis hastarim to be fit for bi'as mayim is only d'rabonon.
The Sidrei Tahara (198:23: d.h. umi'dei avri) brought in my sefer mayim rabim pg. 213 asks on the how can a pasuk consider the beis hastarim a place that must be fit for water but not actually come into contact with water. Since the halacha is that on a Torah level something would only qualify as a chatzitza if you are makpid about AND covers the majority of one's body, it is impossible for there to be a d'oraysa chatzitza in beis hastarim. How then can the pasuk come to exclude beis hastarim?
The Mishna Achrona based on this question concludes that anything that is placed on one's body intentionally can be a chatzitza even on a Torah level even though it only covers a minor part of the body.
Alternatively, the zichron yosef (quoted by sidrei tahara) answers that although chatzitza couldn't be a d'oraysa problem in the beis hastarim because it is only a minority of the body, if one were to close their mouth preventing water from entering that would be a problem. The rationale is that a chatzitza is ta'fel - secondary, to the body, therefore when only covering a minor part of the body, it is as if it isn't there. However, by sealing the lips and preventing the entry of water, it would be a chatzitza if not for the Torah telling us that we don't need actual bi'as mayim (and according to the ritva excludes it on a torah level entirely from even being fit for bi'as mayim). Based on this he rules that although a safeik in chatzitza is a safeik d'rabonon, a safeik about sealing one' lips is a safeik d'oraysa.
The Sidrei Tahara himself says that the question doesn't compel any great chiddush. If not for the pasuk excluding the mouth from requiring contact with water, one would think that water must touch the majority of the body and if the beis hastarim helped make up the majority by the fact that it didn't come into contact with water, it would invalidate the tevila d'oraysa. The pasuk teaches that it doesn't need contact with water, it just needs to be fit for contact with water and would only invalidate the tevila if it weren't possible to come into contact with water.