Yes, yes, the concept as it exists now didn't technically exist to my knowledge back when he was introduced.
That being said, he was the Head of the Jedi Assembly, which functioned in the same capacity as the Jedi High Council that came after it IIRC. The High Council itself being led by both Grandmaster and the Master of the Order.
So, does Head of the Jedi Assembly = Grandmaster?
Especially given we have references to the title of Grandmaster existing well before Odan-Urr's era in the form of Biel Ductavis, I sort of figure we can infer that's what Head of the Jedi Assembly translates into given the concept was introduced and described after Urr was conceived as a character. :hmm
Granted, the era Urr stemmed from also could have conceivably held numerous grandmasters too, given that was only done away with by the Ruusan Reformation IIRC, but eh.
Not to say Urr was the most powerful when he died, much like Satele certainly isn't by the time Barsen'thor and HoT rose to prominence and hit their current peaks, but at the time they were elected they certainly would have been the "greatest" (makes further sense when Fae seems to imply she'd need to give the title up to pass it along anyway, even if she was technically surpassed I assume)
Then again, I'm also a fan of series like Ravemaster where some of the Top Tiers like Haru's father and Lucia's father had shit feats, yet per word of god and omniscient narrative are afforded powerscaling.
There's a nice balance to play when working on connecting the dots :maybe
We know as a Sith Lord he was stronger than any Jedi alive.
Which includes Thon, you know, the same ****er that basically matched the entire power of Ambria's dark side energy by sealing it away in Lake Natth. Same dark side energy kind of razed the planet :maybe
Honestly, Thon has one of the better feats of raw power in the mythos from that too.
Also heard something about Luke considering him one of the darkest beings he's ever met, second to Sidious or something. Never seen the exact quote though, I've only heard that through hearsay :hmm
The quote exists, I think you can find it on here in the Exar Kun respect thread made by ancient power.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."