Correct, minimum is 23 feet (7m), and very similar to many suburb municipalities (it is not just Milton). Builders in the past 5-7 years have gone to this format, selling the whole "wide-lot" angle (which just means shallow in depth).
It's not just in Milton - check lot depths in newer areas of Mississagua, Brampton, Maple, Richmond hill, Markham, etc... many builders are going the route of 80-90ft lot depths on the majority of homes (even 700k places in a high end Thornhill neighbourhood)
Blame us, the buyers, who keep buying them too
It's an industry trend, (think of these lots like the sidesplits of the 2005-2015 era), where government policies are matching builder plans, and we the people keep buying them up.
Gone on the days where everybody gets 65x200 - we don't keep chickens and rabbits and a huge garden like by grandmother did anyway
Back to your question, it's not just Milton, and it's not just Mattamy - it's almost everywhere that you find newer development. Not EVERY builder, but enough that some rare ones can claim they offer fantastic "Traditional" deep lots of 100' or more (even though you still only get a 30' backyard since the layout of your house is long and narrow)