I mean, no one here is ever going to know the truth of it, but writers and producers lie to protect their art/income. It's something that we see all the time. They're not going to come out and say "Hey, our corporate overlords and the McDonald's corporate overlords entered into a marketing agreement worth hundreds of millions of dollars so we had to make a big deal of one of our characters working at a McDonald's and McDonald's in turn is heavily promoting our show my naming a meal after us, putting our logo on their cups, and selling a bunch of our Loki themed merchandise, so Sylvie works in a McDonald's now".
Wherever the idea originated, a writer saying "Hey, we could have Sylvie working fast food in the 80's", a Disney exec, a McDonald's exec, I dunno, but cross promotions like this don't come about just by having a scene set in a fast food restaurant. They're the result of a lot of time and money and market research.
So me, I'm always going to lean on the cynical interpretation when it comes to deals like this involving companies like Disney and McDonald's. To me, it's the more logical/likely case. But I could be wrong.
Edit: lol this reminded me of an exchange on a different forum when another Marvel show came out and some users were talking about the products in it and how they were 100% for character development, because Disney would never be so crass as to accept money for product placement