I think it bears repeating through that Hulk's rebuild was a much more expansive job than Merlin are planning for Nemesis in many ways. And even then the track had been replaced by the start of April, so the rebuild took around 7 months, followed by 4 months for installation of the all-new effects and testing.
It would therefore seem strange if the Nemesis work took anything like as long as Hulk because a) not all the track is being replaced, and in particular the parts remaining in situ are the parts that have most of the mechanics, and b) from the application it doesn't sound like they will be adding anything significantly different to the ride, so there are fewer unknowns to be tested once the ride is rebuilt. And the other factor to take into account is that there is minimal foundation work planned, which is likely to allow things to move much quicker.
All in all, it would seem quite likely that if Nemesis was to close at the end of the season, the work could be completed in time for a 2023 summer season opening in late May. It really doesn't seem like it would be financially sensible to have the ride closed for any longer than it needs to be, considering that they are likely to receive a nice attendance bump whenever it comes back, so it doesn't need to wait for the start of a season or alike to maximise the effect of its re-opening.