It isn't jumping to conclusions. The Thing can assimilate and imitate living matter at a cellular level well enough as to be indistinguishable from its prey once the process is complete (to the point where its prey may not even be aware it has been assimilated until rapid mutation is required).
There is no canon to suggest that the prey is still in there when they've been assimilated and inflicting harm on them to test if it's the thing or not is a quick way to expose the ruse. Xenomorphs express no reluctance to inflict self harm or harm on others.
Xenomorphs have no possible defense against this. Indeed, nothing aside from a similar manner of organism has any defense against this.
Which the Xenomorph is - albeit a slower changing and more stable creature.
The Thing was only vulnerable in the film due to it being in the extremely inconvenient situation of landing on the only region of Earth that lacks consistent life.
Xenomorphs can end planets too, plus they're pretty reliable at getting to other planets alive in one piece. They're also more than capable of functioning in arctic conditions. You can run from Jeffrey, you can't run from Xenomorphs. Being isolated also kept Jeffrey safe - if a single human ever got back to report what happened, Earth would get burned. And probably the human that came back too, along with all the people present just in case.
The Xenomorphs do not need to ever get near to Jeffrey. Jeffrey need only starve as the humans are gobbled up by the Xenomorphs, and the humans fight both. Net loss of biomass all round. Jeffrey needs energy and is less efficient in acquiring nutrition than aliens.
When the Xenomorphs showed up on Earth they had to nuke it. If the thing showed up in a town they'd have to nuke it.
It's an even match with the only difference being time, reflected in Jeffrey's fast changing mutations and the hive mind's calculative and surgical actions.