TAXITIME is set for all flights. In case of a SLOT , when sending a ready message a different TAXITIME can be set. I do not work at a large airport so I cannot be sure what happens there , but an incentive ( slot adherence reports ) exists for ATC to set the correct global TAXI TIME in order not to give takeoff clearance out of CTOT interval.
The burden to be ready at EOBT stands with AO/ground ; the burden to get the aircraft airborne after startup in the correct time interval stands with ATC. Each has its own job
.