But "arrive" isn't very old-fashioned, so translating it with an old-fashioned word would be out of place. You're saying don't use "прийти" in the situation I described, use явиться/появиться, instead? She was going to "appear" today at her new workplace? (obviously the literal English translation failing in this instance)
For example, I would say, "I was going to do that yesterday" in situations where I would be very unlikely to say, "I intended to do that yesterday" or "I planned / was planning to do that yesterday." Certainly the meanings tend to coincide, but "intend" or "plan" add a certain nuance that "going to" lacks.
My impression is that "собираться" is more generalized in use than "plan" or "intend" but not quite so much as "going to," which seems to be further down the path of taking an "actual" verb and "wearing down" its specific meaning to become merely grammatical in nature. I don't think the same phenomenon applies to "plan" or "intend." If you "planned" or "intended" to do something, you really did have to "plan" or "intend" it, but if you were "going to" do something, you need not have actually been "going" anywhere.
(None of this is to opine on the acceptability of "intended" as a translation here; I don't know/recall how that's handled in the course. If it's accepted elsewhere for "собираться," I don't know why it wouldn't be here.)