Why does Mongo Compass give me the error "$add only supports numeric or date types, not string" when I'm not using `$add`

Question

I'm getting a bizarre error in Mongo Compass (v1.2.1) with Mongo version 4.2.8

I have a simple aggregation pipeline step that compares a date and checks an equality.

However I get an unexplainable error saying: "$add only supports numeric or date types, not string"

I'm not even using $add!

Further, if I modify the second part of the expression, I get a different error saying unknown top level operator: $eq, when I can see in the docs that it clearly is