This isn't military backing Biden. This is the top brass backing Biden. The leadership of the military is basically just politicians. Obama spent 8 years removing anyone with an ounce of patriotism. What this letter sounds like to me is the top brass being worried that the actual ground troops are going to back Trump. They are trying to avoid a situation where a majority of the military won't take orders from Biden. It's further proof that the politicians are freaking out since the 6th because they realized they are about to face consequences from the very angry American public for stealing the election.