I would argue that tab Rupturing is AoE. Cleave typically means 2-3, but with Assassination's current setup you can get way more than 2-3 Ruptures rolling. I think the highest I've seen in a dungeon is 7-8 concurrent, and I just happened to have run out of targets at that point. That's AoE in my opinion, it just has a ramp-up time. It's also exponential as more Ruptures means more Venomous Wounds procs which leads to more Ruptures. It's AoE, it's just not burst.
You're definitely right about HFA/Socrethar. Luckily, from what I can see in the Legion tier those fights seem to be less common/important than HFC. Additionally, most dungeons of consequence will be Mythic+ where the mobs are scaling in health rather than scaling in number, so Assassination will have an opportunity to spread Ruptures.
That is not necessarily true about live WoW, but in Legion with Mythic+ you are probably correct.