They didnt 'stumble across' a new setup in 2020. They brought a new suspension in Silverstone2 that enlarged the setup window of the car and then they found better setups for the car that improved the high and medium speed turns performance without the need to add more wings.
Race speed traps are useless but qualy speed traps in Imola and especially Portimao, where the car was also fast in the twisty stuff, show that old engine still has grunt.
Although Ocon and Budkowski say it has lost some competitiveness... which is no surprise since it's more or less an RE19C
Budkowski confirms it's the same 2020 PU
https://www.fia.com/news/f1-2021-portug ... transcript
Q: (Chris Medland – Racer) Marcin, a big focus at the start of this year has been on aero issues and rate concepts across different teams, but the two guys next to you seem to have made bigger changes to their power units than yourself and Mercedes did over the winter. Is that something you’ve seen play out in terms of performance and the pecking order in the early part of the year?
MB: It is true and it’s early days and we all do GPS analysis and competitiveness analysis; the reality is quite tricky to do and you need the data from a few races to build a good picture but yeah, we kept pretty much the same power units than last year. We’ve done some small evolutions to it and that’s less than what our competitors did. It was a strategy, we’re having a completely new power unit for next year and we’re talking about our arbitrating the ’21 car with the ’22 car, well, it’s the same for us. We have limited resources on the engine side and we need to chose our path carefully and we felt that it was a bigger gain to have in terms of producing a whole new power unit for next year and if the sacrifice was to keep the same power unit this year then it was worth doing. As a result, we are suffering a little bit this year in terms of competitiveness but you have to use your resources in the smartest way and that’s the choice we had to do.