No I am not aware of that I guess you should try to find out.
I tried it on the S7 Edge and, unfortunately, I was right that it doesn't work.
adb reboot-bootloader simply reboots the phone, i.e. back to the OS.
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Use my commands from within TWRP?
^This. TWRP's commands and binaries are independent from the ROM (ie it has its own).
I tried it on my tablet and, yes, TWRP correctly resized the image. Thanks.
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.