The Open Park siren doesn’t say which zigbee profile it is using, but since it is sold with other devices using ZHA, that should be OK.
That said, it is considerably quieter than the other models we’ve been discussing, closer to a doorbell than your usual siren. Its maximum is 93 decibels. In contrast, the Aeotec goes to 105 dB, and the Zipato goes all the way to 115 dB (The same as an American residential smoke alarm.) Since this is going to be inside a shed away from the main building, that might make a difference to you. It also appears to only have one sound, as opposed to the Aeotec and the Zipato which both give you a choice of several.
In good news, it is spec’d to operate down to about 14°F. so it is a truly outdoor device.
It’s definitely worth considering. It will probably just look like an on/off switch to smartthings. I didn’t see any advanced features in the product description.
(By the way, all the stuff in the product description about channel management and not interfering will not apply: you have to have a hub which also does that, and smartthings does not.)