Kakashi was alone against six ninjas and had almost no chakra left. How on earth could he had survived if not saved by Minato the moment he used that kunai?
The Body Flicker is fast but covers short distances... The Flying Thunder god technique is instant teleportation.
And what about that part where we see Minato about to fight and he asks the other ninjas if they left some kunais around the battle field? Were they just to warn him as you say? No, he can teleport to the locations where the kunais are.
Kakashi could still live for a short while, and minato is just plain fast.
and the fact it was never mentioned is pretty big against it.
All I've been hearing is "that's just insanely fast, how can someone be that fast, it has to be that jutsu" or "it's a summoning wrapped around the kunai" which have no backing to them.
I'm not saying he didn't use space/time to reach kakashi, but right now you are just proving minato to have even more godly speed, which is fast enough to keep up with them and reach them in an extremely short time. Your side has absolutely no evidence, and that lack of evidence is proving my side apparently.
Just because someone "says" that he can instantly teleport to the kunai via space/time doesn't make it true, like I said it is just a rumour, this jutsu is fake.