Pro hype here, BUT add htmx to it and it's a match made in heaven. Rationale: templ is a template engine with the same syntax as go (so no need to learn or be worried about any new syntax). Htmx makes html more powerful
I don't see how it compares. Go-app does page rendering in the frontend, in this website's case based on Markdown files. It does have some backend generation capabilities (for SEO, apparently) but that's not its main feature.
I suppose you could probably port templ to a frontend framework through WASM, but I don't know why you would go through the effort when go-app is already there.