Your theory is sound. I also think that if he used the phrase “he knew what he signed up for...” or some version of that, she was too ignorant to understand the context. Anyone who has spoken to an ignorant person of color has had this experience. One can never underestimate the thin skin of dumb people!
It’s not an issue of the recipient’s skin color nor ignorance so much as intent and perhaps grief. When that “he knew what...” quote is taken in its full context (which Wilson/ilk conveniently left out), it talks to the soldier’s heroism and nation’s thanks. Now Trump is oft tone deaf and bumbly, but he’s not one to show blatant disrespect to a fallen soldier’s widow and I’m more inclined to believe Gen. Kelly’s testament than a so-called congresswoman that’s been out to get Trump anyway she can.