The idea is quite simple - crawl websites and figure out what makes them tick. Of course you're not going to enter the exact same market. No one is obligated to find a competition-free, low-hanging niche for you to occupy.
A lot of billion dollar company ideas are simple too like Facebook, Salesforce and Yelp. But I doubt 1 person can build those.
Those BuiltWith folks probably have hundreds of regression tests running daily. Maybe a bunch of freelancers who check for newer versions of software too every month.
When he started more than a decade ago, there was zero competition and it was a novel idea. Now there is stiff competition.
This is far from simple - this is actually a very serious project, not an example of 500$ stuff!