Take this $elemMatch predicate:

{a: {$elemMatch: {b: 2, c: 3}}}

It expands to this ABT:

|   PathGet [a]
|   PathTraverse [1]
|   PathComposeM []
|   |   PathComposeA []
|   |   |   PathArr []
|   |   PathObj []
|   PathComposeM []
|   |   PathGet [c] PathTraverse [1] PathCompare [Eq] Const [3]
|   PathGet [b] PathTraverse [1] PathCompare [Eq] Const [2]

The last 3 lines correspond to b: 2, c: 3. This predicate can only be true when applied to an object. So the extra PathObj / PathArr checks are redundant: we can replace them with true / false respectively.

It's not enough to check for a PathGet however. A predicate like b: {$exists: false} can be true when applied to a non-object, because it uses PathDefault which can "catch" Nothing. Other nodes like this include PathConstant and PathLambda.