This chapter was quite immersion-breaking. First, it's hard to believe that a small city in Japan could have enough students to fill four classes. Second, it seems unrealistic that Sayu, without any prior knowledge, could develop a functional game demo in just 40 days.
as someone who studied game dev, seems p plausible, esp with the explanation given:
she used free assets, so the huge timesink of modelling was bypassed
followed tutorials, which if you're geared towards programming, a week is really all you need to really get a feel for it (even that may be much cause of this third point)
she made a first person game, which there is an insane amount of template stuff for, so more than a month is almost excessive if she only made like one small map with enemies and nothing else
tldr she probably made an "asset flip" considering she talked about following tutorials, so p easy to do, even without any knowledge whatsoever
(I hate tutorials personally, but do understand their use: they provide tangible results, even if try don't really always "teach you" what you're doing and the code behind it)