I have no idea what this means. What makes web apps not serious?
If you won't take that seriously, then what?
I'm sorry but to inject a dose of reality, that's a very low bar.
The guy is making a web app which other people want to pay for, looks like just about the right height for a bar, and I'd say not low at all.
"Tech Company" is looking to hire someone that knows technologies A, B, and C to use to make a product that's also intended to make money (allegedly).
If that's not good enough, I ask again, what is? What do you really want in a prospective junior developer?
I realize some people do cutting edge programming, but the bulk of us probably spend much of the day shifting strings and filling in model fields and chasing webpack configs. Or maybe I'm just a sap and everyone else is doing cool stuff idk.
If in the process you learned about something you could actually go up and do an interesting lightning talk about, that'd be worth it. Maybe you learned something about a specific industry and the challenges that developing something for it has that other industries don't have - maybe you accidentally spent weeks/months deep-diving into the technical details of HTTP/3 or some browser API and worked out you could do something that nobody has done before outside of Google.