I've found this thread viewtopic.php?f=42&t=3058&p=15306&hilit=maxitems#p15306 from quite a few years ago, where Turley says
Is it still correct? Does the MaxItems value is ignored for items in subcontainers?...After this update:
every container can hold up to 3200 items (fixed core value)
if you add something to a subcontainer core checks for weight and itemcount inside the subcontainer, for every parentcontainer it checks for itemdesc.maxweight