In this instance, the <article> elements should not be articles: they’re not complete, standalone items. To be sure, as https://html.spec.whatwg.org/multipage/sections.html#article... says, there’s subjectivity in deciding whether to use <section> or <article>, but I’m content to say that <article> is wrong here.
(Last time I looked into one of these classless CSS starter files, it was <aside> being used for cards, which was very wrong.)
In a case like this, I’d prefer the inner elements to be <section> or <div>. Or better still, just start using custom element names which are perfectly valid, e.g. <grid-card>. There, no class. I will permit you to claim that that’s cheating and avoiding the point of classlessness; I don’t mind.
But the big pitfall of trying to go fully classless (if you interpret that as also meaning you can’t use your own custom element names) is when HTML just doesn’t have tags for the semantic concepts you’re trying to express. You want a quote, table numbered list? Yeah, we’ve got tags for those things, go knock yourself out. But you want a card? Here, have an article within a section. … what? That ain’t semantic. How’s an author supposed to guess that those tags will present like that? That’s what classes or autonomous custom elements are good for, indicating intent.