> parallel hardware interface

The physical layer as described in report 1822 is bit-serial, but uses synchronous clocking scheme without fixed baud-rate reminiscent of parallel SCSI's REQ/ACK. It is a weird scheme for serial links, but it has the clear advantage that it adapts to cable length automatically (but runs at significantly smaller bitrate that could be supported by the cable, which probably was not exactly an issue in 70's).

In fact the whole report 1822 deals mostly in bits (the messages on the IMP-host interface can have arbitrary bit lengths). Obviously this was done in order to support architectures with weird word sizes (eg. 36bit PDP-10).