The 4040 has a very narrow range of permissible clock cycles in the datasheet (1.35-2 µs), which makes me think it uses dynamic logic, so the tolerance for higher/lower clocks is likely quite limited.
I also had an idea to overclock i4040 more, but even 740kHz was hard to work with, because I had just 200 stm32 ticks per i4040 sub-cycle to do all I/O logic (RAM/ROM interface for i4040).
I can imagine Intel to just have specified a narrow operating range where the device is guaranteed to work reliably within the specified voltage/power envelope. We might be surprised to see higher clock speeds if we play with the supply voltage for example.