Hi, jazzbiker.
Yes, tce-update compares md5 of the extension but does not compare the .dep files.
Either of these would have fixed my issue of missing libzstd:
1) Keep the Xorg-7.7-3d.tcz.dep file that my system downloaded when it upgraded that extension (which listed zstd.tcz as a dependency), fix the issue that zstd.tcz.dep was missing
-or-
2) Download juanito's corrected Xorg-7.7-3d.tcz.dep file (which lists libzstd.tcz as a dependency)
After either of the above, "tce-update" (or "tce-audit fetchmissing") would have discovered that libzstd.tcz was missing from my system and would have fixed my problem.
Since "tce-update" cannot detect missing or changed .dep files, I was out of luck. It's a bug with low probability of affecting a user, but I was affected by it twice in this scenario. Lucky me!