> JSON doesn’t have floats, it has numbers, and how they’re used after being parsed is not part of the spec.

I think that's the problem they were trying to describe. Without a formal spec, systems won't agree on how to handle floats. JS engines treat numbers as 53 bit signed floats, so passing a well defined decimal there through JSON means losing precision at the edges.

Money stored in integers gets around the issue by simple virtue of not really needing more than 53 bits to accurately represent the values anyone is going to encounter.

There are downsides like all the extra math or functions to handle doing the math everywhere money is manipulated or displayed, but this is the sort of thing where static typing is really helpful, and isn't too hard for juniors to understand that they should always use money functions to work with money data.