Yes, it's real and valid. (And I know that sentence sounds like a TRA but hear me out.)

Labels are valid when they communicate some useful information about something. When someone says they're heteroflexible I get a good idea of what they mean: They are mostly heterosexual but may occasionally have homosexual urges or behaviors. There's nothing wrong with that, and there's nothing wrong with putting a label to it.

Labels are bad when they are so esoteric that they need to be defined nearly every time they're used, or when they lack a consistent definition altogether and can be anything. All the stargenders are useless because they fall into at least one, and usually both, of those categories.

I also think the "bisexual" label is a bit questionable around the edges, the Kinsey 1s and 5s. Sure you can say that a bisexual is anyone who isn't completely hetero or homosexual, but that covers a lot of ground. Is a man who has only ever had hetero relationships and watches almost only hetero porn, but a couple of times a year looks at a picture of a twink just to get started (but can't actually finish with that so switches back to women) still a bisexual? Does it really communicate anything useful for him to call himself that? Are we applying a "one drop" (of precum) rule here? I'd call him "mostly straight", but with a just a little bit of homosexual attraction.

I want to reinforce this point: Labels aren't inherently bad. It's about getting useful concepts condensed into widely-known words. I don't see a problem with hetero/homoflexible being used for the Kinsey 1s and 5s.

Of course there are also people who would use these labels out of shame and to hide what they really are, but that's a more general problem. Lots of gays and lesbians call themselves "bi" for years because they don't want to admit, often to themselves, that they are gay.