JSON explicitly does not require floating point (https://tools.ietf.org/html/rfc8259#page-7). As a data-interchange format, it's not interested in most of the issues surrounding floating point numbers (operations, rounding, exception handling, etc), but it doesn't even use the interchange formats of floating point either.

I'm sure that as a practical matter, it will be a rare implementation that does not work by deserializing non-integer decimal numbers into floating point numbers, but the JMAP spec does not require this behavior, and I believe it uses no decimals.