“There are no clear alternatives, some less dedicated users are returning to tumbler”. Tell me you aren’t paying attention without telling me you aren’t paying attention
Poor research could also be a reason. I just went to Google to search "Alternatives to Reddit" and the only immediate result that I found to mention Lemmy was a report off Android Authority with the headline "Looking for a Reddit Alternative? Lemmy tell you, they currently kinda suck"
So... Not great results off that one. I'm not really sure where they got the line about Tumblr, but I could absolutely see a writer not well researched on the topic making the statement made in the OP article about there not currently being a clear alternative.