"'Birthing people' clearly refers to the mothers"
No it doesn't "clearly" refer to mothers at all, any more than "birthing partners" clearly refers to the people you have listed.
Why did you feel the need to define "birthing partners" and list who it might apply to, other than to ensure that everyone understood who you are talking about? 🤷♀️
(Reading this back, I think I might be coming over as preachy and/or grumpy - apologies if so. Just rather rushed!)
There is the same need to explain who "birthing people" are because it is not necessarily obvious from context that the term is intended to refer to mothers.
"Birthing people" could just as easily be interpreted as either a colloquial term or "professional" jargon that is intended to encompass just about everyone closely involved in the birthing process apart from the mother.
IMHO the only thing that is clear is that close to 100% of mothers and expectant mothers understand the same thing by the word "mother".
Trans ideology and the surrogacy industry go hand-in-hand, shoe-horning misogynistic terminology into language, globally dehumanising women and commodifying reproductive processes.
Who is the "mother" in the surrogacy industry? Not the woman who is the "birthing person". Not the woman who is the "gestational carrier".
Prior to this linguistic flipperoo a surrogate mother was always the woman (usually a woman) who substituted for the actual mother in caring for and raising a child long-term. Often the child's aunt, grandmother, other female relative or female friend in informal "fostering" or "adoption" arrangements.
This meaning still exists, but now only in relation to animals, as this thread illustrates:
https://x.com/HairyLeggdHarpy/status/1378661064339042306
Threadreader:
https://threadreaderapp.com/thread/1378661064339042306.html
(Both of the above have been archived if they should disappear - new readers, check the usual places.)
So much more to say on this if I only had time.