Mercedes (well Toto) has already said that they expect cars to be slower for the first part of this year due to the downforce and tire changes:
https://www.autosport.com/f1/news/15437 ... -this-year
But that's coming from the fastest car in the grid, the rest have margin for improvement. So it isn't that farfetched to believe that certain teams (McLaren included) can be faster than they were in 2020 from the get-go. In the special case of McLaren, lets assume the tire + downforce cutoff takes away up to 0.5% of laptime. If the new power unit brings 0.5% improvement (that's from power + the packaging benefits) as these experts are assuming (they say 0.5 seconds, but 0.5% might turn out to be less than 0.5 seconds on some tracks, so I guess on average it's within that range), then that means that whatever aero development that McLaren has made over the winter, and potential improvements to the whole package in general will translate to direct pace improvement compared to Mercedes.
McLaren managed to cut off ~0.3% to Mercedes from 2019 to 2020. But Mercedes had a monster of a car in 2020. Compared to themselves, McLaren improved at around 0.7% over their 2019 car. Considering the restrictions for this year, that 0.7% perhaps is a little too optimistic. But assuming the best case scenario, McLaren improves by 0.5% on the chassis side from their 2020 car. That translates to 0.5% of net gain compared to their 2020 car (after taking off the losses from the rule changes + the assumed improvements from the power unit).
Since Mercedes thinks they will be slower than 2020 initially this year, that means that we can expect Mercedes to only make an improvement of 0.3-0.4% in 2021 from their own car in 2020 (which after taking off the assumed 0.5% from the new rules, that leaves them 0.1% slower). In this scenario, McLaren ends up 0.6-0.7% behind Mercedes at the start of 2021 ...
To put that number in perspective, that would mean that McLaren at the start of 2021 can potentially be better than RedBull was on average throughout 2020 compared to Mercedes.
That's why that 0.5s number seems way too exaggerated in my opinion. Don't get me wrong, I hope it is true. But realistically speaking, we can't really expect McLaren to make that huge of a jump.