Yes that is correct. We could attempt to detect all plugins by pinging known files but that would involve indexing all available free plugins and doing around 40,000 HTTP request on the site you want to check ;) Glad you like it!
Not really, indexing the top 100 most popular plugins would be enough, and you don't have to ping all files of each one, a single file for each one would be enough (so 100 request).