I had the same thought. Probably bad phrasing, the author might not be a native English speaker.

It means that Osewa is open-source. I suppose that its possible for SSH alternatives to exist that are not open-source, which is why its worded as "An open-source alternative to...".

no- native speaker here (Spanish mother tongue), I read it as "an alternative, that is open source, to ...", no implication that the other options are not open source