Probably--most of the v1 devices that aren't Wi-Fi are a proprietary ZigBee protocol, and I think this is what most people who've tried them report. It is, I guess, a little promising that Hubitat sees anything from it at all. From what I've read, the Iris v2 hub has only one radio for the two ZigBee profiles, so if Hubitat feels like it and the open-source release Iris plans on making happens to show how this ZigBee profile works, it's possible they could add support. I wouldn't count on it, however (unless they happen to think the appeal to Iris users--or the generally cheap cost of v1 sensors to the rest of us--is worth it).
The first sentence is true, but I think the second part sentence is worth clarifying. There are three (or three and a half, depending on how you count) generations of most Iris devices--e.g., the v1 motion sensor (won't work), the v2 (model 3326-L and minor update 3326-L2), and the short-lived v3 (model iL07). The v2 is ZigBee HA 1.2 (most boxes say this but I think the "L2" models stopped printing this on the box), and I think v3 is ZigBee 3.0. Both of these generations work with Hubitat.
Also, most (all?) Z-Wave devices from Iris should work on Hubitat. I see you mentioned the siren. I'm not sure if there was ever a "v1" siren, but the only one I've seen was the Utilitech siren that was labeled "Works with Iris." That one is Z-Wave and does work on Hubitat. It also appears the Orbit hose timer, assuming there wasn't a proprietary "v1," is supported--see this thread: Orbit Iris Hose Faucet Water Timer - #3 by chuck.schwer. (My guess is that "Works with Iris" as opposed to early "real" Iris-branded devices are standard ZigBee or Z-Wave devices that should work on other platforms.)