This is an interesting one.
Do you/they have more on William and Sarah? Enough to create profiles for them? Have they reviewed their wills? Are there other situations where Poyntz is called Pine?
Does the Owsley group research tangents to their own surname, like her parents would be, or are they just happy to have a wife who married into the Owsleys name?
So, if I were you, I'd want to know that they actually do all they can to research the women's lines, too, and in this case that they/you have done all you can to confirm or deny their theory (by tracking down all the info on William and Sarah, and exploring this 'nickname' theory). It sounds like there are potentially a few puzzle pieces still out there and they may be making a conclusion a little early.
(With the last question regarding tangents off of a male line, I've often been surprised by how focused researchers are on their name and the male line. They have worked for years on their surname and have an incredible amount of expertise. When you ask them about the rest of their tree - everything but their one male line - they've never really looked into it. Because of that bias among genealogists, I typically start with the women and have found that is where the interesting discoveries are usually found.)