I would think so, but my read of the (really, really old) linked topic is that even with single quotes it wasn’t working.
Yes. Discourse-setup puts double-quotes around the password. If pups and/or docker do the right thing with
DISCOURSE_SMTP_PASSWORD: "hise@#ijflkjfpassword"
then it should work. N.B. that discourse-setup strips that comment about the #. If we want that comment then we need to put it on standalone.yml on the line above it.