The comment I quoted claimed that for a language to take off it needs first class Windows support.

If we’re going to limit the discussion to languages which “can be used for writing Mac, Linux, Windows apps”, then the argument is pointless. By definition, any language which can be used to write Windows apps has Windows support. That’s circular logic.

To address the argument, it only makes sense to discuss languages which don’t have¹ first class Windows support. Everything else is irrelevant.

¹ Or didn’t but were still popular at that time.

Swift has support for server web apps (https://vapor.codes/), so I assume it has some interoperability with other operating systems.