hi,

Both PSEN and !RD needed for accessing external code memory WITH a movx instruction."

Still not right. /PSEN is activated only during either MOVC or program fetch. This signal is the read strobe for external code memory chip.
Both /RD and /WR are used as read/write strobes during external data memory chip access.

By the way: there is another difference between /PSEN and /RD: the duration of /PSEN is twice shorter than one of /RD.

I had based it on Fig.16 on page 13 of the following PDF.

Yes, and so you have read there next:

Figure 16 shows the signals and timing involved in program fetches when the Program Memory is external. If an access to external Data Memory occurs, as shown in Figure 16b, two PSENs are skipped, because the address and data bus are being used for the Data Memory access.

By other words, Picture 16b shows one /PSEN for command fetch from external Code Memory and then one /RD for reading access of external Data Memory. Basis concept assumes them as two separate chips (generally: code - ROM; data - RAM).

Regards,
Oleg