To follow up on Gaile's comment, you might consider the lack of fields for baptism or burial dates, or the inability to enter date ranges, design flaws in the WikiTree database. Leadership has told us repeatedly that they won't add baptism or burial fields. I don't think they have answered about adding or changing radio buttons, but I would not expect such changes any time soon. Just my opinion; they could prove me wrong any second.
I prefer to think of these things as current factual circumstances that we have to work with, rather than as design flaws. Sometimes it takes a little flexibility to maintain accuracy while working within the provided framework. WikiTree has a field for birth date. The best or only source you have is a baptism record, so you know the person could not have been born after that date. If you insist on entering the baptism date in the birth date field as your estimate, and no other date will serve, then the 'before' radio button creates a problem. That is a current factual circumstance, a minor obstacle to overcome.
You can demand that WikiTree modify its code to eliminate the problem, and wait patiently for that to happen, or you can adapt. Enter the date one day after the baptism, and click 'before' with a clear conscience. Your profile is as accurate as possible and consistent with its sources, and WikiTree's design has not invalidated anything.
The counter-argument seems to be, "But that's not the baptism date." I agree, it's not. It's an estimate of the birth date, based on a baptism record. Don't confuse the source with the event. If potential reader confusion over the one-day difference concerns you, please consider that your readers might be able to grasp the concept of 'before.' And of course you can, and should, always spell it out in the bio.