but how do you know that.. like i said have they tried segwit on the testnet and then had other non-segwit implementations on the same chain to see how they react..
like i said unless they are going to try everything they cannot promise everything
They will be. Prior to deployment on the mainnet, segwit will be deployed on the testnet. I do not know whether they have had non-segwit nodes on the segnet chain. It is fairly trivial though to implement a node to run on segnet and see if it works.
If you think that they aren't going to test everything, why don't you help out and test things yourself? It is all publicly available.
If I sold you a car radio, and claimed that it was 'backwards compatible' with your car, and then later you read the manual and discovered that it causes your airbags to no longer work, and you'd have to buy some new airbags from me as well just to be safe and secure again - you wouldn't call my radio 'compatible' with your car, would you?
Clearly that isn't backwards compatible, but that is not an analogy for segwit.