Though the support rep did not discuss the details
Well that's a shame, because details are exactly what you need to confirm this one way or the other.
Your question to support seems to have skirted around the issue a little and mentioned "creating assets", which is not the issue here - the issue is you using the Spine runtimes to render your animations and distribute them.
If someone wrote a DragonBones extension that would be fine, if someone wrote an extension that rendered .json files from DragonBones or even Spine itself which didn't use the official runtimes - that would also be fine (although owning Spine and then not using the official runtimes would be an odd choice)
As I said the issue is using the Spine Runtimes (which GM does) to render your animations and distribute your games without a licence to do so.
Game Maker's license with Spine clearly extends to its end users much in the same way that my players do not need to purchase a Game Maker license just to play my games
Of course not, because nowhere in the GM EULA does it state that a GM licence is required for an end user to play a game made with GM, the Spine EULA clearly states that a Spine licence is required to use the "Runtimes".
Dragonbones files are allowed, but they cannot provide official support.
Yes, DragonBone files are .json, they are allowed, and I assume they mean they won't offer any support for DragonBones and/or getting DragonBones files working with GM.
What isn't allowed is using the Spine Runtimes to render the .json files and display your animations in your game, without a Spine licence.
There is a post regarding this on the
Spine forums here, where Nate clearly states as much.
I'm a Spine Pro user, so if you would like me to email Nate and get it confirmed one way or the other then let me know - Nate is a good guy and I'm sure would have no problem responding and clearing the issue up.
Might also be worth mentioned some of the YYG staff to maybe chime in, as at the end of the day nobody will know except them and Nate.
It's possible they have some kind of deal where a licence to use the runtimes is included in your GM licence, and although I doubt it I'm happy to be proven wrong.
I'm not saying I'm right, I'm just thinking about it logically - but if you want to risk all the hard work you might put into a game for the sake of saving $69 then that's up to you.
PS. Spine is awesome, I highly recommend it.