I genuinely don't see anything wrong with saying he knew what he signed up for. As multiple people have pointed out, the statement highlights the bravery of this and every fallen soldier.
It's a cold comfort that will be better appreciated by those with a military background. Also, it's ludicrous when offered by a draft dodger like Trump who did his best to avoid signing up to begin with.
In this situation,
any comfort will be cold.
I suspect that Ms. Johnson knew her husband well enough to understand that.