It's not a tag, there aren't even HTML/XML tags in Typst. The effect of that <figure> is giving the #figure(...) element the label `figure` (i.e. the name is what's inside the <>). Probably using the label "figure" was not the best choice for an example, something like <my_figure> would have been a bit less confusing showing that it is an arbitrary name/id that you choose.
Edit: and I think you also misinterpreted the other comment about the <div>. It wasn't about using <div> in place of <figure>, but rather that using <figure> in typst does the same thing as id="figure" in HTML.
Edit: and I think you also misinterpreted the other comment about the <div>. It wasn't about using <div> in place of <figure>, but rather that using <figure> in typst does the same thing as id="figure" in HTML.