The problem with looking at a current marketshare number like that is that it doesn't take into account trends. In recent weeks since Sony corrected their production issues and stock skyrocketed, PS5 has been outselling Xbox Series by something like 5:1 in the UK in recent weeks. Depending on how long that trend holds for, the lifetime marketshare could to rapidly shift in Sony's favor, by the end of this year it will likely be closer to 65/35 in favor of PS5 even with a sales boost from Starfield and a likely price cut on Series S for the Holiday, and if the trend of PS5 substantially outselling XS continues into 2024 it could be lower than 70/30 by the end of 2024 possibly.
CMA need to look at trends, not just the current marketshare.
We don't really know how much PS5 is beating the Xbox Series in last couple of months in the UK. Yes PS5 has received massive gains YoY but we don't know how far behind it was last year where Xbox won Jan/Feb 2022. It's definitely not as bad as 5:1 though.
Absolute worse case scenario for Xbox in February (by assuming they only outsold PS5 by 1 unit last year) is 3.6:1, can't work out January unfortunately as no Xbox YoY data but as PS5 improved 98% in January and not 316% like February it's probably close to 2:1 at worst.
Don't know if there's anyway to see UK estimates for VGChartz anymore. It's on the legacy weekly charts but that's it.
You're right they should be looking at trends though.