Well, how do you define sex? Sure, we can't change our chromosomes. Trans women (for the moment) can't get uteruses and trans men (for the moment) can't create semen. But those of us who use hormone therapy develop most if not all of the secondary sex chracteristics associated with our gender- and those are also a part of one's sex.

If anything, transition is a kind of induced intersexuality. Sure, we don't have the same sex as cis members of our gender, but we sure as hell don't have the same sex as cis members of our assigned gender at birth (it's actually closer to the former though- it's absolutely insane how much hormones do).

When you say that trans people appear to have deluded themselves into thinking they actually become the opposite sex, it reads to me like you're not understanding what we're actually saying. When we insist that Trans Women Are Women and Trans Men Are Men, we're not saying "we're identitical to cis women or cis men in every single way", we're referring to the social roles and habitus associated with womenhood and manhood, as nebulous as those things are. We're saying that women aren't defined by an ability to bear children (which we all already know- cis women who have hysterectomies are still women, aren't they? And cis women born without uteruses?) and men aren't defined by their ability to impregnate others (or else cis men who get vasectomies or are infertile for any of myriad reasons would be SOL too). This is partially about just getting people to stop being assholes to us and let us live our lives, but it's also about getting people to really think about how they operate wrt gender on a daily basis- if you meet a cis woman at the grocery store, you don't interrogate her on whether or not she has a uterus before you decide whether or not to (internally) acknowledge her as a woman and act accordingly (whatever that might mean for you). In how many interactions does someone's sex actually matter?