Like a lot of other things, that was completely ignored for his revival.That wasn't his first appearance. When he first appeared back in silver age he was explicitly immune to red sun energy:
Red Sunlight from a variety of sources has worked on him.
It's true. This is perhaps the most disappointing thing about Prime. On the one hand, he has moved enough planets to alter the center of the Universe and has gone toe to toe with Ion, a major reality warper as well as slaughtering Green Lanterns and even Sinestro Corps at the same time left and right. He even took a direct hit from Black Adam without flinching. Like even a little.
OTOH, he's been punched out by Conner Kent. A lot.
Makes no sense.
That's not the Ion he fought.and has gone toe to toe with Ion, a major reality warper as well