Superman instantly depowering and then instantly gaining back all his power really depends on the writer. In early, mid and late post crisis, losing his powers under a red sun was a slow, gradual, process. It was not the instant thing it is today. But ever since Geoff Johns wrote Secret Origins, Superman has been instantly depowered if he was exposed to a red sun, then instantly powered up again if he came under the influence of a yellow sun.
I prefer the old way of things, but those days are gone...
As for the issue itself, I found it likeable enough but Superman should have known he'd have no powers on Krypton even before going. So, time traveling to a place where Superman knows he won't have powers was pretty reckless and just a little stupid. He's lucky he didn't get stuck on Krypton minutes before it exploded because then he would have died.
So Superman is now being written as if he is Mon El under a Red Sun? Likke you said, until now, pretty mh was that Superman still stayed super untill all of the Solar energy in his body cells was used up, as he could not re energize under a red un, but still coiuld go hours to days as superman!