No one has ever prefaced such a statement with "for your purposes." There is a reason for that.

It actually occurs fairly often. A good reason to prefix such a statement with "for your purposes", is to indicate that modelling a statement as true is effective for achieving your purposes, without getting into a more complex discussion of whether it actually is true or not.

For example, "for your purposes, the movement of objects is described by Newtonian physics". The statement after "for your purposes" is ill-defined (what exactly does 'described' mean?) as an actual claim about the universe, but the sentence as a whole is a useful empirical and falsifiable statement, saying that you can assume Newtonian physics are accurate enough for whatever you're currently doing.

As a second example, it might be true to say to an individual walker arriving at a bridge, "for your purposes, the bridge is safe to walk over", while for the purposes of a parade organiser, they cannot simply model the bridge as safe to walk over, but may need to consider weight tolerances and think in terms of more precise statements about what the bridge can support.

For your purposes as a human being in a typical situation who doesn't want to signal negative things about and to any and all transgender people, you should behave in line with gender identity as innate. It is a reasonable piece of advice relating to social etiquette in this area.

It isn't necessary to get into demonstrating the probable truth of this (including breaking down the definition of 'innate') to give this advice and the original quote decided to avoid starting that argument, which seems like a reasonable call.

Statements like this do make some assumptions about what your purposes are- in the bridge example I gave, the speaker is assuming the walker is not a parade organiser considering leading a parade over the bridge. Such assumptions and guesses about the audience's purposes are unavoidable when giving advice, though, and this particular one seems quite reasonable. In no case do these assumptions "subjugate" you to make them correct.