The problem [he] describe[s] is nonexistent. [He] say[s] <div class="card big"> is bad because

    some other CSS can come along
    and co-opt what big means

To me the complaint is like saying, when programming, that I shouldn’t define functions with names in a file that’s imported into another file, because “some other” file can “come along” and redefine those names. If that’s a problem then that’s your problem, not the existence of classes. Or names.