---------- Post added at 23:19 ---------- Previous post was at 23:01 ----------
I'm aware that TWRP has its own binaries, but the image resizing that SuperSU 2.76 natively attempts is performed, as far as I can tell, by /sbin/launch_daemonsu.sh.
Looking inside that file, resize2fs is called without a path, so whichever $PATH is in effect at the time is the one that is used. That presumably results in the ineffective invocation of /system/bin/resize2fs on these Samsung devices.
I don't have TWRP on my S7 Edge, so I'll have to either install it or look up how to create a flashable ZIP for FlashFire if I want to resize the image on that device as well.
Actually, now that I think about it, TWRP won't even work on the S7 Edge, because /data is encrypted.