I used to handwave cloud portability. Turns out when you're shipping things and need extra services and you have deadlines, you build against the platform. I think the GP comment was probably expressing wariness of the free cloudflare tier that entices you to build against their APIs and their product shape in a way that inevitably locks you in. Sure, you could migrate, but that's expensive.
Yeah, good point. For a little hobbyist site of no importance, I'm not too worried about vendor lock-in, but that calculus changes as it gets more important.