Some of what he said might have been and some parts probably weren't.
> Cyrus it's pretty obvious he's outright lying
He was telling a story from the perspective of the (or some of them) the Greeks might have seen. It's just as likely to have been hearsay as outright intentional lies.
> And incidentally, Herodotus is the ONLY ancient source that so much as mentions Tomyris, full stop
How many other ancient sources do we actually have on some of the periods (especially related to Persia) described by Herodotus? Also as far as we can tell the narrative history or even Mesopotamian/Asyrian/Babylonian style chronicles weren't really a thing in ancient Persia so it's not inconceivable that he just wrote down one of the oral stories coming from there (it probably wasn't that clear to the Persians themselves what might have happened to one of their previous rulers after a generation or two).
Overall by the standards of ancient historians Herodotus was probably above average.