We can no doubt expect the remaining 390s to be finished much quicker than the first ones are. The same has happened with other rail fleets, and the same happens with aircraft too.
The closest recent comparison to this refurbishment I can think of is the 2010-2015 refurbishment of the Tyne & Wear Metro fleet, which involved the same level of strip out as the 390 refurbishment (and if anything was actually more intensive work, as it also included corrosion repair). The first unit in that refurbishment took 15 months to complete, as opposed to the scheduled 9 months. The time the work took reduced from 15 months per unit at the start of the project, to just 8 weeks for the majority of subsequent units, and the program was completed 6 months early despite the initial delay. Of course this isn’t by any means an identical comparison, there are many differences between the two projects, but it’s a good example of how the first unit in a major refurbishment program running several months late doesn’t always equate to the remaining units also taking much longer than planned to complete.
Interesting that the Modern Railways article mentions that the new seat design came from outside the rail industry, does this mean these won’t be the same seats as First used for Lumo after all? As those seats were designed by Transcal, who are very much established in the train seat market. Or was this just an error with the article?