Always more to find
This is in fact how weighting works, which is explained in the OP including links to Antar's original FAQ about weighting. It's clearly not just two good players using Steelix if you look at the moveset stats. Multiple people around 1630 Glicko and higher were using some combination of Steelix/Scyther/Roserade/Incineroar/Keldeo/Sylveon/Dhelmise/Gardevoir/Drapion on the ladder during the dozen days after DLC. Steelix was probably not a terrible choice given that these players were winning.
You would see similar anomalies for a given Pokemon no matter what Glicko threshold you wanted to use for weighting. When Mesprit moved up from PU to NU last gen, it had 6.28% usage in the 1630 stats and almost 17% usage in the 1760 stats. This gen's higher usage cutoff wouldn't even have "saved" it that month or any month after that until the end of Gen 7 because it stayed well over 5% in 1630 and well over 10% in 1760.
If Steelix becomes unviable in UU with all the new drops from OU then it will fall back down to RU, likely next month since it's sitting around 1.8% again as of a few days ago.