It is backwards compatible. If the current implementations that already exist chose not to use segwit, they would be fine and can continue to function. What I meant is that if those implementations want to support segwit, it is up to them to implement it in their own software without any bugs. They can follow the publicly available documents that specify the exact changes that segwit makes.
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?