Nedarim 11a - T'nai Kaful

The gemara seems to assume that the machlokes of whether you need a t'nai kaful is really an argument whether we can deduce from the language that he used what he really means to say (m'chlal lav ata shomeia hein). Why does the gemara understand the din of t'nai kaful to be a general principal in how we interpret a persons language and whether we can derive what he means by his implication, maybe it is a gezeiras hakasuv in laws of tenaim that they condition has no power to hold back the action that was done unless it follows a specific format i.e. kaful? The Steipler seems to ask this question but asks it slightly differently. The Steipler asks, since we find by a neder that a yad is sufficient and we are entitled to some level of interpretation, so why don't we use m'chlal lav ata shomeia hein, as a "yad"? The Steipler explains that since we find in Tosafos in a few places that even R' Meir agrees that we don't always require t'nai kaful, so long as the circumstances indicate his intent (sometimes he doesn't even have to speak out the condition at all), if by nedarim it would have the status of a yad, then in other areas it should have the status of circumstantial evidence and be a binding condition even without repeating both sides. It must be that the requirement of t'nai kaful is that, although we can normally use the circumstance to help understand his intent, the Torah does not allow us to use inference to interpret his intent. Once we see that there is not an absolute requirement by every t'nai to be kaful, there is no reason to assume that this is a gezeiras hakasuv of t'nai, rather we assume that this is a general concept that although circumstances may be used to release one from requiring t'nai kaful, nevertheless inference cannot be used to understand a persons intent - we don't make an inference from la'v to hein or from hein to l'av. So by nedarim as well we do not use michlal la'av ata shomeia hein and it would not qualify as a yad mochiach to what he means.