In my Python script, when I set the param upsert: True, I got some errors for importing “The user already exist and upsert parameter is false”. When I changed it to upsert: "true", it works as expected, ignoring the existing records. I guess this param is not respected as a boolean but simply checking a string value of "true".
I tested your use case and believe what you said is correct. If upsert is set to anything else except for 'true', it will be evaluated as default, that is, 'upsert':'false'.
