Let's say Superman is at
point 1 in time and wants to go to
point 10 in time. He implies that the timeline being corrupted is the reason as to why he can't do that (He later reveals this is not the case, but we will ignore that for now). So for Superman to have Immeasurable speed, he would need to reach
point 10 in time and react to it at Immeasurable speed, but the timeline would need to
change points in time around, to the point where Superman ends up in, say,
point 7 in time. But the timeline would need to do this at Immeasurable speed, or else a Superman with Immeasurable speed would overcome its "corruption" and land in the point in time he wants.
And this obviously requires a lot of speculation. The timeline being corrupted is a poor excuse; Superman saying that at all goes to show he doesn't react to his time travel at Immeasurable speed (on top of the other statement he gives).
The same way one can be "strong enough" to punch the ground, activate a distant volcano, but not destroy the area around oneself. The physics and logic are different. Superman's speed triggers Time Travel, and that means he can choose any era he wants, as if by magic.
Mind you, this is not more or less logical than the idea of "Immeasurable speed" itself. They're both the fantasy of doing something normal humans can't, however:
- Fiction often creates Time Travel with some level of science to back up its logic to a small degree. This is why we see so many speed-based Time Travel, because going faster than light warps time. As to how that justifies characters in fiction knowing where they're going? Well, it doesn't. They just know it. That's just something they can do.
- "Time Travel as an ability" and "Time Travel via Immeasurable speed" are still not the same, because the more likely interpretation requires the smallest assumptions, and Immeasurable speed is an infinitely higher assumption, times 2. So it requires strong evidence (Obviously, I'm not saying it's impossible). But in this case specifically, Superman reveals he doesn't know where he's going and where he will end up when time travelling.
Hope you find this reply satisfactory to your question.