It’s a bit misleading but the dealer is speaking part truth and part fudging the issue.
‘Some’ of the particles get burned off as part of the regeneration cycle, so a longer, hotter, higher rpm journey will burn off more, and slower, colder stop-starts will see them build up. Eventually they build up too high, trip the warning lights and off you have to trot to the dealer.
They are right that the motorway-type journeys help, but are not the panacea - the net result is always an increase so you’re just delaying the inevitable - the things clog eventually anyway.
I’ve had several vans (DPF) which have been mostly motorway use with very few stop-start journeys, so essentially ideal conditions, and the filters have always blocked and needed work eventually. That repair costs a fair whack too.
That is something the EV guys never have to worry about to be fair.