Not a 9, but I'd like throw my 2 cents. Feel free to correct, dispute, add.
Because 9s want to avoid conflict or anything which upsets their inner peace, they develop both permeable and sometimes thick boundaries (seemingly contradictory).
Permeable in that it can show up as a tendency to take all sides of an argument, and conflating values and ideas of those they're comfortable with as their own.
It's easier this way as having an actual identity/opinion of your own would require personal investment, exposing oneself to the various faculties of conflict: disagreements, judgement, aggression.
There's some definition overlap between sx instinct and e9 merging, but my understanding, albeit wholly impression-based, sees sx instinct as more visceral, restless, and volatile, whereas e9 merging is shrouded in vaguery, inertness, melting. Like the song "Fade into You" - Hope Sandoval, a 9 fixer.
Back to boundaries, I think it can be thick in that there's a stubborn denial or forgetfulness of anything negative or upsetting (perhaps more wing 8). Wing 1 is more fussy/bitchy about anything negative or upsetting, so it's more permeable, or easily affected.