The article you linked was updated, looks like he did have a gun and was spotted while aiming through the fence:
Investigators found the suspect had left behind his “AK-style” rifle, as well as two backpacks, one of which contained ceramic tile, at the scene, Bradshaw said.
We have a little more information now and it is interesting:
https://nypost.com/2024/09/15/us-news/alleged-would-be-trump-assassin-ryan-routh-donated-exclusively-to-democrats-since-2019/
The article starts with:
Then later on it mentions: