Lemmy doesn’t seem to get much recognition in the wider Fediverse - it tends to get bundled as part of ‘other apps’. Mastodon is much bigger, so better integration with Lemmy probably gets deprioritised below their own issues and feature requests (e.g. I was reading today that Markdown support is often requested, but the base version still doesn’t have it)
From his Mastodon, it looks like OP (Cory Doctorow if it’s not showing in whatever app you’re using) is doing talks in Serbia at the same event as Bruce Sterling, so it’s as likely that he wrote the ALT text for the image himself.