I'm of the opinion that the test itself is bogus, but that the theory it is attempting to test has merit and is worthy of investigation with better tests. Test-retest reliability doesn't distibguish between those two possibilities.

I always get damn near the middle of the chart, on both axes.

But I think this confirms your claim in my case. For nearly every single answer on the test, my response is "it depends." I really could go either way on nearly every single question because I need more context.

It makes me think the whole test is essentially BS, kind of like a horoscope where you can read whatever you want into it.

Yeah, the test isn't really very accurate. And if you're getting close on both axes that mean the test is actually telling you it isn't sure! (this doesn't mean your personality is actually close - it means the test doesn't know).

There are better tests (like this one https://sakinorva.net/functions) which will give you a confidence percentage for each cognitive function rather than just a best guess overall type. But none of them are that great, and currently the gold standard for determining type is to only use the test results as starting to point and to familiarise yourself with the theory to fine tune that, potentially with the guidance of an type expert.