Hey thanks for reading, I'm the author of the piece. The original went through a couple rounds of edits since it's on the company page but PM if you want the original :)
Oh I see. Popular as in number of software installs, rather than development effort, amount of data under management, amount of developer mindshare building and maintaining apps against it, that kind of thing. Fair but generally SQLite and MySQL are not considered the same class of engine so the wording could have been slightly more clear "most popular open source database server" or similar