Ouch! He really chose the hard way to deal with the keyboard there.
That kit he bought is almost certainly using an MCU to scan the pseudo-keyboard. Just using an MCU to directly scan the MSX keyboard would save all that soldering, and would also allow him to use key combinations to generate codes for keys that aren't on the MSX keyboard, such as F6-F12.
Besides the fact it's a shame for that MSX1 that has been murdered, it's also nonsense.
This way you can turn anything into a modern gaming machine. A C64, dishwasher or shoebox. Whatever...