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.