No. Idiomatic means "the normal way of doing things" basically. So in that context they are trying to say "et cetera" is the normal way to say "and so on" even though it's not English. It's a big stretch though. You wouldn't use "idiomatic" to describe that text unless you really wanted to find a word starting with i.
What you're saying seems to agree with my point. If something is "the normal way of doing things", why would it be "set off from the normal text". You're saying idomatic means normal, whereas MDN says it means "set off from the normal", aka abnormal.
