Weird RC time constants for possible debouncing
It is for EMI purposes.

Even though a single row is scanned every 4.5ms or so (about 220Hz) if the scope picture in the service manual is accurate (it may not be), there are 16 scan outputs (15 used), so the scanning happens at 3.5 MHz rate (my estimate, could be wrong, most likely it is slower given the RC time constant of 1 MHz and the pulse shape on service manual) where one wire goes down and another goes up. Given the physical size of the unit, each of the scanning signals also have considerable length inside the unit to form an antenna, whether on wires between PCBs or copper traces on PCB.

Most often the overlooked part is not the scanning rate of roughly 3.5 MHz, but the edge rate of the signal. Basically, the sharper the edges of the pulse, the higher frequencies it must have. Infinitely fast sharp edge would in theory have infinite bandwidth. That is why there is slew rate limiting on the scan output wires in the form of a RC filter. 1kohm and what seems to be 1nF indeed has cutoff at 1 MHz.

It limits the bandwidth of the scanning signal so it does not radiate much.