The blogger uses Big-C Classical Big-M Music, which almost always means European classical music, pre-1900 [0]. You won't find many non-white composers in that particular group.
Capitalization matters, it creates a proper name out of something whether we agree on that name's appropriateness or not (Classical Music being so narrowly defined could be considered a misnomer, as you've pointed out there's a lot more classical music out there).
[0] I couldn't remember when I initially wrote this when the Classical Period ended, it ended earlier in the 1800s than I realized, by the common definition apparently 1820. The start was 1730 (using the earliest time people use to be generous) so "Classical Music" defines a period of less than one century of, specifically, European music.
