Even more than that, the page is cached as it was at the time it was shared. I've seen this happen with documents that were later edited, with hilarious results.
Isn't that the case with "unfurling" anything, though? Whether Slack generates a thumbnail or just pulls text from meta tags? Same with other apps like Teams, FB Messenger, etc? None of this is known to poll for changes frequently enough to avoid the hilarity of caching.