It is quite a popular opinion in the space. To quote past me from various sites and point in the timeline, you can find my overly long definitions and separations below. But it is something I often encounter.
The long rants
I think what would help your discussion is if you separate Bionicle and Constraction. I understand they’re intertwined and often used interchangeably, but it is what causes a lot of conflict.
Constraction: A buildable action figure. Examples include Throwbots, Bionicle, Galidor, Knights Kingdom II, Hero Factory, Ben 10, etc.
Bionicle: An action-story theme from 2001–2010 and/or 2015-2016 primarily consisting of constraction sets.
Constraction is generic, Bionicle is more specific. Constraction can use any pieces and building system for its purposes, even those outside of the dedicated ones for it. Bionicle is largely based on an aesthetic built around specialized pieces made for its run; the setting and how it looks is wholly dependent on the sets and canon. There’s an expectation of what a Bionicle character or creature should look like and failure to meet that often brings disappointment. Can you use a lot of custom Technic or System builds and recreate the look of Bionicle’s specialized parts? Certainly, but plenty will still wonder why you bothered.
I use this distinction due to certain crossover of items. Bionicle had System playsets, I wouldn’t say those aren’t Bionicle. So while Bionicle is largely constraction, it isn’t fully beholden to it. In a similar vain, just because you can create some bust using CCBS or parts made specifically for constraction doesn’t make it a constraction build. If it was from a character or thing from Bionicle, it can be claimed to be a Bionicle bust, regardless of parts used, but simply using what were parts made for Bionicle would not.
Constraction, per its name, should only be concerned with creating some sort of buildable action figure (or action figure adjacent, e.g. vehicles). So that goal in of itself, which is largely foreign to typical Lego builds, will generate the unusual or innovative builds that username seems to appreciate. And that really shouldn’t be limited to just Bionicle.
Where it can get really iffy, outside of other subgroups within Lego with custom parts, painting, cutting, gluing, etc., are poseable models and figures. So something like Gundams I wouldn’t count as non-Lego constraction, as most wouldn’t survive the purposes an action figure is made to perform. But do mechs and mechas count in general, since they share a lot of the same qualities? Or the old Technic Star Wars figures, are they just display models or do they feature enough action functions to count as an action figure? Up to the beholder I suppose.
It’s more here are labels that work better in distinguishing purpose/build types and edge cases that make the definitions difficult. Bionicle has a harder line than Constraction, but that only matters if the builder is trying to keep within its established rules.