The sexologists who've studied TIMs and TIFs closely say that whilst some report of a change in sexual orientation after "transition," these are merely reports and don't necessarily correspond with reality.

What's more, these reports of a change in sexual orientation are much more common amongst straight men and straight women who get on the trans train than amongst gay men who do so.

Finally, the research has not shown that these reported changes are caused by taking cross-sex hormones, androgen blockers, getting surgeries or any other trans-related treatment or events (like changing documents, going public with the new gender identity, changing type of clothing worn, etc).

We investigated changes in self-reported sexual orientation and the relation to important life events in a large cohort of transsexual persons in Germany by means of qualitative and quantitative data. We could show that self-reported changes in sexual orientation are frequent in transsexual persons especially in originally gynephilic (straight) MtF as well as androphilic (straight) FtM. It was hypothesized before that change of sexual orientation might be influenced by hormonal therapy or SRS.

But these researchers found that

reported changes of sexual orientation are not particularly associated with any transition event. Thus our data challenge the view that either hormonal therapy or SRS or any other event has a direct influence on self-reported sexual orientation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192544/

Ray Blanchard calls the reported change in sexual orientation that some TIMs claim to experience "pseudobisexuality."

The Oxford dictionary definition of "pseudo" is: "(adjective) not genuine; spurious or sham."

The Oxford Thesaurus says the synonyms for "pseudo" are "bogus, sham, phony, imitation, artificial, mock, ersatz, quasi-, fake, feigned, pretended, false, faux, spurious, counterfeit, fraudulent, deceptive, misleading, assumed, contrived, affected, insincere; informal pretend, put-on, fakey; and British informal cod."

TL;DR version: people who base their entire identity on denying reality, pretending to be what they are not and bullshitting aren't the most trustworthy group, so their every utterance shouldn't be taken as the gospel truth. And guys of the lady dick variety are especially likely to tell cock and bull stories - IRL and on anonymous social media like reddit.