That is probably true but as long as you do not press a key on both the original keyboard AND the PS/2 that is not an issue. I suspect the signals are logical AND towards the bus so if internal would put 00000001 and external would put 00000010 the MSX would read 00000011 which would still be okay but give a different character then expected.
Keyboard input is pulled up, so it is normally vcc when keys are open, that means every time a key is pressed PPI will feed current to the cartridge adapter that will force a 0 on a line PPI is outputting 1,if it just drain enough current to have voltage level detected as logical 0 by z80 but still within the ppi limits, it should be fine, otherwise, it might cause issues to the PPI in the long run.
