It's a high risk method. But it's much more effective than traditional job applications. So if you weigh the time spent (10-20 hours building custom thing for company) vs spending 10 hours filling out 30 applications, I would guess this is a better approach.
It's also a method described in "guerrilla marketing for job hunters". There's others in the book if you're looking for a little lower effort non-traditional methods.
I put a much higher priority on the people I work with. So I would add to that risk the scenario where I get the job but it turns out to be a bunch of bros. Maybe there was some public insight to the kind of people this person would be joining to mitigate that risk.
I spent a lot of time learning everything I could about the company, the people, their product, etc. I knew half the engineers by name from various videos they had posted and was fully convinced it was a company of amazingly talented people.
Just curious -- were there other companies you put this much effort into studying and making such a stellar first impression? Did you feel sure enough you would get this position that you didn't need to?
When you're looking for work and need to get a job (e.g. to support a family) spending 10-20 hours working on a single job opportunity is risky because they may just say "sorry, we like you but _____". where _____= {there's a corporate headcount freeze, we don't need that skill, we already have something that does that better, we're not good at hiring}. I still think it's likely a good strategy, just worth acknowledging the inherent risk.
If you have oodles of free time, then sure, it's low risk.
It's also a method described in "guerrilla marketing for job hunters". There's others in the book if you're looking for a little lower effort non-traditional methods.