It's perhaps worth stressing that the error condition was just added so the difference could be seen without looking at the typeset output: it's not really related to the underlying issue.
This version runs without error without setting the suppress error flag \ifcsname T\ifincsname e\else E\fi X\endcsname AAA\else zzz\fi \bye but typesets zzz with luatex and AAA with pdftex/xetex David
