It is absolutely a lie, because web components can handle arbitrary data just at much as any framework.
You're holding web components to a higher standard here in expecting them to take arbitrary data in HTML when HTML itself doesn't support arbitrary data. Notably you can't assign arbitrary data to framework components from within HTML either, so how are web components any more limited?
The original comment was that "all HTML attrs must have string keys and values", which is completely true.
The point of web components is that they create normal HTML elements. So it makes sense to consider what the value of them being normal HTML elements is. You can write them directly in your HTML source code, for example. But if you do that, you only get access to attributes and not to properties, and therefore everything needs to be strings. Alternatively, you can treat them as DOM nodes in Javascript, at which point you get access to properties and can use non-string values, but now you've got to deal with the DOM API, which is verbose and imperative, and makes declarative templating difficult.
Yes, we could compare them to components from other frameworks, but that's honestly an absurd comparison. They're simply trying to do different things. Frameworks aren't trying to create HTML elements. They're trying to reactively template the DOM. It's just a completely different goal altogether. The comparison doesn't make sense.
You're holding web components to a higher standard here in expecting them to take arbitrary data in HTML when HTML itself doesn't support arbitrary data. Notably you can't assign arbitrary data to framework components from within HTML either, so how are web components any more limited?