the =! is only intended to work with strings which is why stuff like
scores=!

{Joe=1..5}
don't work in selectors
since the value for nbt is not a string using the =! should not work yet
nbt=!{NoGravity:1b}
works fine in a selector

e.g.
/kill @e[scores=!{Joe=1..5}

]
errors
/kill @e[nbt=!

{NoGravity:1b}

]
doesn't error