The TransActual website has information about transphobia:
https://transactual.org.uk/transphobia/
I picked out a few highlights:
The denial of self-determination ... constitutes transphobia. For example the denial that trans women are women, trans men are men and that non-binary people’s identities are valid and should be respected
Using biological essentialism to try and delegitimise trans people....[is transphobia]
“Man”, “Woman” and “Non-binary person” are social/cultural statuses. Trans people have existed for millennia throughout history and in every part of the world. Consequently, trans people have as much right to claim their genders based on biology or otherwise as cis people do. The Endocrine Society states that there is “a durable biological underpinning to gender identity”. That being said, bio-essentialism plays into the hands of extreme right-wing ideologies.
Deliberate misgendering ... is abuse. Calling trans women, “men” or trans men “women”, or non-binary people “men” or “women” is transphobia. Using the wrong pronouns, such as “she” for trans men and “he” for trans women is misgendering. Not using “they/them” (or similar) pronouns for non-binary people is transphobic as is using these terms for binary trans people.
Treating trans people any differently from those who are also the same gender is transphobia. For example, treating trans women as different from cis women is discrimination and has been defined as such in the UK’s Equality Act (2010).
I emboldened my favourite bits. So I guess the answer to OP is ...yes?
No explanation. Just, because we said so.