According to the column by Jolyon Palmer, Ferrari made a mistake, they did the undercut hoping to get ahead of Hamilton not expecting that Vettel would ever make up the 3.5 seconds needed to get ahead of Leclerc. Somehow the unexpected happened and Vettel did make up that time, whether he was exceptionally fast or Leclerc was slow or a bit of both, who knows? Once Vettel was ahead with Leclerc in second it wasn't really in their interest to ask Vettel to move over, especially when Vettel needed the win more than Leclerc in some ways.as i put on the other thread. for vettel to be told to push & leclerc not. is very unethical and behaviour you would expect from different teams
Maybe they should have, but it's a judgement call and that moment in time they chose not to for whatever reason. By allowing Vettel to remain where he was they got a 1-2 finish which is an excellent result for them and not what they expected at that track. It hasn't helped the tension between their drivers, especially after Monza where Vettel gave Leclerc a tow in qualifying then got caught out when he didn't get one in return as agreed. It's easy for us to chew it over afterwards and say they should have done this or that, if they'd asked Vettel to have moved over they'd have upset him, by not asking him to move over they upset Leclerc, there was no third option that would satisfy them both sadly.