Definitely a case of HUACA in my opinion
While the fare rule is ambiguous, I have never heard of an agent trying to impose such a restriction
The ambiguity is that in one place the rule refers to Open Jaw and in another to Surface Segments, as follows:
0. APPLICATION AND OTHER CONDITIONS
First / Business / Economy RTW / Open Jaw travel via AA/AY/BA/CX/IB/JJ/JL/KA/LA/MH/NU/QF/QR/RJ/S7/UL/XL/4M operated services worldwide.
and then later:
4(c) Travel may originate at any point for which fares are published and must terminate at the same point, except that origin-destination
surface segments are permitted as follows:
a. within the country of origin
b. within the Middle East
c. between the United States and Canada
d. between HKG and China
e. between Malaysia and SIN
f. within Africa
g. between Maldives and Sri Lanka/India
I personally have had four RTWs with origin-destination open jaws. All were 16 segment tickets (of course!). Never had a problem; not when purchasing and not when making changes.
And BTW, one of them was a BA-issued ticket