I saw that in the thread too @Regulus. But it makes no sense to me - Firstly, why would boosted people show signs of infection quicker? Surely protection from the booster would slow things down, not speed them up? But secondly, if they did "start their clock" earlier, that ought to mean they clear the infection earlier, not later
I think the thinking is "symptoms are response to infection from the body, therefore robust response, earlier symptoms, therefore earlier testing meaning earlier discovery, but regardless of that initial response it's still 7 days to ditch the virus." so because it was detected on day 1 rather than day 3 without the vaccine you get the leaving early problem.
I too struggle with this as a theory, why would an earlier, stronger immune response not shorten the time of infection.
It does fit with the antigenic sin theory (strong immune response but slightly off the mark so takes longer) but that's not a well supported theory. This is where I'd like more info compared to the "maybe the boosted were weaker" option, as it very much informs further boosters or not.
The good news of course is that any of the vaccines, boosted or not, are so far still very successful in preventing serious illness of the immune system overload method that harmed most.