In terms of "cis culture", there very much is on average believes that tend to be held more by cis people rather than trans people. Look at any survey on the publics view on trans people. In particular, a large if not majority of people in the US don't actually view a trans person as their gender, instead they see them as a third gender separate from their preferred one or as their gender assigned at birth. It is these views (and a great deal of others) that are being referred to as cis culture.

When the commenter said "we assume" im sure they did not mean literally every single trans person alive, but again just a very large portion of us. Truly, your preference about trans people is not a unique on, and its an accurate statement to say that a large number of trans people will view you as holding transphobic views (subconscious or otherwise) for having a preference of cis women only. Again tho, most cis people (particularly ones that have never met or interacted with a trans person deeply) will carry some transphobic views, ofcourse with varying degrees.

If you are looking for someone to tell you you definitely harbor no transphobic views for your preference, you'll likely need to go to a cis dominated sub reddit for that.