I like when scientific theory is presented this way. It is self-evident that it is scientific theory: anything in science can be prefixed with "We have evidence that suggests that..." You must follow that formalism in some forums, such as publications, but when having a coffee table discussion, when someone says "Mars formed around the same time as Earth", mentally add a note that says "We think..."

The nugget of truth in your statement is that some assumptions we are more sure of than others. We can probably say how old Mars is with more certainty than how much water there was on there. That may or may not need to be clarified, depending on what the focus of the conversation is.