All that means is that one is far less likely to meet too many lazy unmotivated students at Ivy/peer elite schools…not necessarily that they do not exist.
After all, there are always a few exceptions to the rule.
The fact your kids didn’t meet anyone who is lazy/unmotivated is likely due to the fact they gravitated towards the critical mass who are just as motivated/hardworking as they are,
However, lazy and/or unmotivated Ivy/peer elite students do exist. Especially considering some may not have been such when they applied/entered as people can and do change due to setbacks or life-changing events in the intervening period.
Another thing to keep in mind is that a few of the lazy and unmotivated may be able to skate by and even excel at an Ivy/peer elite precisely because their level of academic giftedness/acumen is high enough that they could slack off and still do well academically.
To some extent, this was how some of my salutatorian HS friend’s MIT roommates described him considering he never pulled an all-nighter, slept more than they did, and seemed to have much more free time to enjoy all the campus/college parties and leisurely pleasures that Boston/Cambridge could offer than they did…and yet he managed to graduate near the top of his BS and MS classes in EE within 4 years and after a stint as a working engineer in Europe, returned to MIT to complete an EE PhD.
They were all struck by a mix of awe with some tinge of jealousy at how he made it all seem effortless compared with their own experiences.