Hm, that SISTRIX article shows some graphs indicating that some websites received more traffic from Google directly after they changed from subdomains to subdirectories. It then tries to explain what exactly might be the cause for this (given that Google says that they don’t prioritize one over the other). And the the two explanations mentioned are:
- some systems do a better job at interlinking between subdirectories than between subdomains
Neither of these mean that subdirectories automatically are better for SEO. The novelty mechanism may work in the sort term, but (by definition) the effect will wear off in the medium and long term. (And, notably, it wont work at all for an entirely new forum.)
Which leaves us with the systems argument and that - I assume, correct me if I’m wrong - doesn’t apply to discourse.
Ergo: SEO is not a reason for diverging from the default subdomain setup of discourse.