it is rendered as an ordinary link because the website obviously doesn’t support OpenGraph. In cases where that happens, I tend to change the text so that the link becomes an inline link. And in that case it renders as a mini-onebox: https://cispe.cloud/publicregister/ Nice!
So I’m wondering: wouldn’t it make sense if the ordinary onebox would fall back on mini-onebox whenever no meta-data is available?
The space trick is simple and very useful… once you know it. I had noted that (some? all?) external URLs show the page title when placed in the middle of a paragraph, but I hadn’t made an association to exploiting this feature as standalone link with an extra space.