This I'm not so sure about. It will certainly be different, but it's still an open question whether it will be easier to secure from a national security standpoint.

There's the physical security aspect of this, which is as you say, hard to take down when it's decentralized. However, as power generation gets more distributed, we'll naturally start seeing more (if not most) of these pieces of equipment be controlled over networks (private or public), and securing distributed infrastructure from a software standpoint is still a hard problem.

Just look at the state of IoT security today. It's quite bad, and that's not even realistically including nation state attackers in the threat model. I don't expect this to go well with a decentralized grid, at least not for a while initially.