UCU has been promoting transracialism as well as transabelism since as long as I can remember. The statement that the union supports enabling members to self-identify, whether that is being black, disabled, LGBT+ or women has been a part of its core established doctrine and is never challenged by others in the union, particularly those who are black.
I think it's because UCU has determined that the natural evolution of transgenderism, which allows (mostly) white men to self-identify as women, is to allow white men to self-identify as black. The capability of, predominantly, white males to colonise other spaces, whether they be those of women or black people appears to be a key stance of the union and its senior members, and is paramount over any other considerations.
Transracialism is often seen as a contentious subject for discussion, but UCU have been following its pro-transracialism line for so long without serious challenge, that I reckon it reflects the views of many of the (predominantly white) academics and students who profess a hatred of Dr. Stock. UCU is much more of union to reflect their views rather than Kathleen's. If there was a union for 'whiteism' then UCU would likely be closest to fulfilling that description.
I don't reckon the UCU's support for transracialism is influenced by the two key examples that are always trundled-out when discussing transracialism - Rachel Dolezal and Senator Elizabeth Warren, but rather it's a recognition by UCU that their vision of 'inclusion and diversity' can only be achieved when, most notably, white males will be able to claim to be both women and black, with the option of self-declaring as disabled in addition.
In the face of that, it really isn't a surprise that they have identified Dr. Stock as an enemy; if you are an advocate for rampant transgenderism at the cost of women and girl's rights and transracialism at the cost of black peoples identity and heritage, then 'Doc Stock' is always likely to be someone you would be fearful of.