Has she ever hit anyone that moves as fast as light ray with matter manip that complicated? And he is a new god. The power it would require to change his actual cellular structure would be astounding.
The distances he traveled trumps the way anyone looks at it. He has enough feats to support he's FTL.
Writers ignoring science is nothing new. We wouldn't have super heroes or comics or sci fi, etc if they didn't. They're comics, no explanation is needed.
When he's shown flying fast enough that the artist actually includes the word "Boom" and he's shown exiting a tear in space then the debate is over. On panel feats>>>>>>anything else including your speculation. You not liking a feat or trying to rationalize with real world science doesn't negate what the comics show, period.
I don't know when, what context or what circumstances where Superman stated he couldn't go FTL. But the feats have been consistent that he's FTL for the past 5 years.
So unless you're saying the writers are placing the Sun, Vega, the Source Wall all within a close proximity of the DC Earth, and you have proof of this, then distances are pretty standard to what we know.
What you want to think, believe, hope for, want to happen is irrelevant. Prove me wrong, which you can't because scans are readily available, or concede the point and move on.