Then he must be the World's Strongest Man! No one else can do that!
As you have succinctly put it, strength is the ability to generate force against an external object. Presumably you meant "an external object" to mean ANY kind of object, not exclusively a 28mm steel shaft centered 8.75” off the ground.
As such, using deadlift as the ONLY test would only make sense if the deadlift reliably predicted ability to exert force on ANY kind of object, but we know it doesn’t. Stuart Jamieson is the best example I can think of to illustrate ( YouTube ). He’s deadlifted 630 lbs. But he also competes in the 132 lbs class and can’t bench 200 lbs. Any SS post novice could beat him at literally any strength test apart from deadlift.
So, while I know you don’t enjoy such flights of fancy, I can sympathize with the OP for wanting to have a more rounded test that wouldn’t declare someone like Mr. Jamieson stronger than a 240 lb SS devotees who “only” deadlift 600, squats 500, benches 400, and could put more weight overhead with one hand than Jamieson could with two.
As I have succinctly put it, strength is the ability to produce force against an external resistance. I have not stipulated as to the object being forced. Can a man who deadlifts 900 produce more force than a man who deadlifts 800? Or 630? This is just not that complicated.
Interestingly enough, at the World's Ultimate Strongman event in Dubai, all the competitors deadlifted over 400kg. It's table stakes now. (Yes, it was with straps and stuff, but still pretty cool.)
Mateusz Kieliszkowski Gets Victory At 2019 World's Ultimate Strongman Competition – Fitness Volt
The Stoltman brothers are definitely ones to watch for the future.
Also, UK WSM competitor Graham Hicks is doing Big Dogs 4 pro-powerlifting meet, so that should be interesting to see.