In the documentation, the list of types supporting conversion to boolean does not match the implementation.
https://docs.mongodb.com/manual/reference/operator/aggregation/convert/
https://docs.mongodb.com/manual/reference/operator/aggregation/toBool/#mongodb-expression-exp.-toBool

All missing types except Undefined return true unconditionally :
https://github.com/10gen/mongo/blob/2fa18e5051b2c3487a9bc5fb8a38a96cdf52dbc0/src/mongo/db/pipeline/expression.cpp#L6129

Please let me know if you need more details.