Its an extremely well worded letter.
He is saying his experience shaped his views on international affairs. An obvious truth.
He is saying someone else might be more aligned with Trump's views. Also something that is likely true.
He is not explicitly saying his views are incompatible with Trump's. However, the media will read into it as if that is the case (they already have) because optically they want to make it look like Trump is a tyrant and no one can get along with him, etc.
Trump and Mattis both know this. Bait.
Mattis can move into another role. MSM has to expend ammunition guessing and targetting the next Sec Def. This is how Trump and the military control the narrative and play 4d chess