Board Thread:Questions and Answers/@comment-24520051-20190207035200/@comment-918732-20190611213221

That is it!

In that story, Clark, Lois, Jimmy and Perry are transported to an alien world where Clark loses his powers, but still must become Superman (and cleverly fake his powers) to protect the others. No explanation is given for the power loss, but the world does have a red sun...

Coincidentally (or maybe not), the second story in that issue ("Supergirl's Greatest Victory!") is the first story where Superman explains (to Supergirl, and to us readers) that many of their powers are due to Earth's yellow sun, and they would lose them under a red sun like Krypton's -- and that's why Supergirl and her fellow Argo City-ians didn't have superpowers when they were floating around on a rock -- they were still in Krypton's red sun system.

So, even though the first story did not explicitly state that the red sun is why Superman was powerless, the second story make it clear that is what happened.