To be fair, this is the first year that Imperial and Warwick Maths use TMUA in place of MAT in the admissions process. So they have no previous stats that they can lean on. The case for Warwick is more peculiar. They know that candidates generally would firm Imperial over Warwick. So they need to see what TMUA threshold Imperial is using before Warwick can determine its own. If Warwick sets its Step threshold too close to Imperial’s TMUA threshold, Warwick would run the risk of not having enough people in their class. If it sets it too low, this would have negative implications on Warwick Maths’ brand. I do agree that Warwick knows what its Step threshold is now, but they do not want to disclose it formally yet - just to give them some flexibility if they need.
I have seen Imperial Maths has given out some offers (no step) with TMUA at least 6.0. My hunch is that the Step threshold for Warwick Maths would be 5.0-5.3 area.