Instance owners tend to focus on making a sound backend that scales effectively. Im guessing SEO is not high on the priorities for instance owners. It takes time and is a mix of an art and science and it isn't really a 1 size fits all.
With that being said, I think the SEO will largely be on the front-end devs who build 3rd part apps that run on the various instances. IMO this makes more sense since front-ends can connect to all the various instances available so it's higher up the funnel.
Source: Experience working in SEO + developer of the lemmy web app Quiblr