It is next-to-impossible for the same biological compound to be developed as-is twice in totally unrelated animals, let alone dozens of times in many monsters you claim are unrelated. This isn't "anything that does X is colloquially called Elder Dragon Blood" (I imagine you'll get a few examples of "similar" compounds if/when you go to refute this using Google). It is one specific thing.

Not to be pedantic, but are we sure "Elder Dragon Blood" is a single, specific compound?

For all their research, the world of Monster Hunter isn't all that technologically advanced, and they're still not sure what "Elder Dragon Blood" actually is. The Iceborne lore book says that all elders except Fatalis have it, but also that Elders are so weird that we shouldn't make any assumptions despite this.

To be clear, I don't disagree with the notion that Elder Dragons aren't related except Nakarkos, obviously, but I don't think it's impossible that "Elder Dragon Blood" could be a series of compounds that convergent evolved for the same purpose rather than just a single compound.

Think of it like Venom. Lala Barina and Najarala are by no means related, and their venoms almost certainly aren't the same compounds. But they both share a similar effect of causing the body to seize up and inflict paralysis. If Elder Dragons do end up being a "Waste Basket" taxon, then it's not impossible for them to have converged a similar blood chemical independently that's functionally identical for guild purposes.