Is the almost-24-bit limit related to the fact that 32-bit floats have 24 bits of significand? (cf. https://en.wikipedia.org/wiki/Single-precision_floating-poin...)
Now SVG… try defining a view box up where float precision drops, and interesting things happen.
As in n+1 == n once you go past 2^24 on a float and here they are at 2^25-1. So it doesn't quite make sense as a reason to me.
There's a post above that browsers divide pixels into 1/64th of a unit which accounts for 6bits which puts this limit at precisely that of a signed 32bit integer. This makes much much more sense than the significand of a float.