(Welcome, Hullins!)
No, it doesn't matter in a deep sense, but a bit-serial memory is saving pins, in this technology, whereas a bit-serial ALU would also save transistors. A nibble-serial CPU is a good choice for a decimal machine: it's progressively harder to do decimal arithmetic as a CPU gets wider.
(I'm interested to know whether "
operating in a serial-parallel mode" was an unambiguous and well-recognised phrase back in the day. There was a time when memory had to be serial, and the CPU probably was too. Then there was a time when parallel (core) memory made sense, but flip-flops were expensive so a bit-serial CPU made sense.)