Speaking as a very very casual gun enthusiast myself - - I think it's a tricky subject. Guns & ammo are great, a million rounds certainly seems excessive, and idk it's possible this guy was a black market arms dealer for very very bad people.
When you have guns, you wonder how you'd react to a knock on the door & an attempted gun confiscation. I don't see many scenarios playing out where violence is called for; they are not (directly) threatening my life, but rather confiscating tool(s) that can be used for hunting, recreation, and yes preserving my life in self-defense. Very not cool. But it's still technically not a physical threat to me. If I were to pop off some guns in defiance of a gun collection attempt, that would lend credence to the idea that I'm an "unstable person" that "shouldn't be allowed" to own firearms. Also, my fight isn't with the guy doing the confiscating. He's a member of my community, he's just some guy doing what he's told, maybe he's got a wife & kids. What is to be gained from shooting him in the face? Does that not make me a monster? Maybe this guy thinks similarly, he was confronted without a shot being fired.
No, from one red-blooded American to another, the no-conflict response is wisest & best. Tell them a warrant is needed, when they can't find what they're looking for, give them the ol' classic "lost the guns in a terrible boating accident" line. They will be forced to accept it & move the fuck on. When tyranny reigns, defiance is duty, avoidance/lies/concealment are justified.
My line of thought is this: you can have twenty safes full of badass guns & ammunition in your basement. But that doesn't matter if you've got a gun to your head on your front porch. What is practical? What is reasonable? What is necessary? Just a handful of nice guns made ready & accessible, a daily carry you're familiar with, a solid 12-ga, a .223 hunting rifle, and a few thousand rounds of ammo for each caliber you own.
My gut tells me this guy wasn't a prepper, if his ungodly massive stores of firearms & munitions were so easily found & rounded up. At least not a good one. My gut tells me this guy was involved in the illegal arms trade, he had a setup in his home that no-gooders could visit & "shop" for what they needed.