According to the specification, the permitted content of a "p" tag is the set called "Phrasing Content".
Phrasing Content includes the "math" tag.
See
https://developer.mozilla.org/en-US/...rasing_content
So whomever or whatever said a "p" tag can not include a "math" tag is simply wrong according to that spec.
Perhaps this is an extra requirement by BoD, but it is not a epub or html5/whatwg requirement.