One of the weird things is that people cite their pronouns as subject/object, e.g. she/her, he/him, they/them - which is unnecessary because as long as you speak English, you already know the object form corresponding to each subject form. So it would always be sufficient to say that your pronoun is she, or that it's he, etc. Otherwise, why not go the whole hog and say that your pronouns are she/her/her/hers/herself? (Whether the second "her" is a possessive pronoun or a possessive adjective, or better described as a possessive determiner, is a matter of dispute among linguists.)
So, when we hear "she/they", which is in the same format as "she/her", the natural thing would be to assume that it means that the speaker wants to combine "she" (subject form) with "they" (for the object form, even though "they" is normally a subject form, its object form being "them"). If this were the intent then it would explain why people give their pronouns as "she/her" rather than just "she", and as "he/him" rather than just "he".
But in fact of course(?), as others have said, "she/they" means that the person is equally happy being identified as she (that is, she/her/her/hers/herself) as they are being identified as they (that is, they/them/their/theirs/themself/-ves).