Theist: one who believes in God.
Evolutionist: one who believes in evolution.
Nope, no contradiction there.
It is aximoronic in that it implies a creationless deity
No, just a deity that didn't do fiat creation of species.
... contradictory to the Biblical record, essentially reducing that record to the status of myth.
But obviously pointing out that Genesis is a myth isn't the same as saying that there's no God.
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One might with more justice complain that "theistic creationist" is an oxymoron. You are apparently
defining God as a being who did something that verifiably didn't happen, just as though you defined God as "the being who makes everything in the world bright pink". One can be certain that
that God doesn't exist, and so you are in effect defining him out of existence.
(Of course, you do not know this, just as a blind man wouldn't know that the "bright pink" definition would render belief in God untenable to the sighted. But such is the case.)