"The 25c clock" seemed like a snappier title than "A simple hack that lets an Apple II keep track of real time (while it's powered on) and do some basic multitasking" even though that would have been more accurate.
But then how do you measure time since boot? With simple hacks like this of course. Also, this hack allowed a form of multi-tasking, an 8-bit feature quite well targeted back then.
Consistency was key in such clocks, not necessarily accuracy. The fact of the drift is not important, since it was consistent and could be accomodated for in software - as many peripherals of the day, indeed, required.