Hmm, I guess 'extrovert' is an acceptable spelling nowadays, even though it's the wrong Latin root.
Both "extrovert" and "extravert" are correct. Contemporary language use does not always care about latin, however sorry I am for that. But the author of this post goes even further, and uses both spellings in the same sentence! In extravert-introvert pairs, extroverts report on greater closeness than introverts That can't be right...
In my accent, at least, I pronounce the phrase 'extr_vert-introvert' with an open A, contrasting to the pursed O of introvert.
But if I say the word 'extr_verts', by itself, I pronounce it with an O, identical to 'introvert'.
It's difficult because for me, at least, in my accent, both are degrading to an unstressed vowel anyway, so it's a fine gradation.