Disagree. Firstly there aren’t many situations in fluid dynamics where there is a steady constraint on mass flow. Velocity just increases. (The exception is choked flow from a nozzle IIRC)

Now think about what happens when 6000 people/hr leave the station but arrive on 12 trains over the course of the hour, i.e. in surges. Each peak demand will comfortably exceed the flow rate of escalator.

You get a queue; the equivalent in fluid dynamics would be an increase in density upstream of a choked nozzle at Mach 1 but how to turn that into a relevant metric for eg queue length?

Thats also the reason - I suspect - that in many Tube stations two escalators go up and only one goes down.