This seems to be the right-wing's main angle of attack on this issue. Seeing how forcefully they're pushing this fact across social media I'm kind of resistant to going along with any narrative they are trying to build from it until more facts come out.
I saw the interview with the guy who said he saw the shooter and he came off as kind of a doofus, he more or less said the extent they went to alert anyone was just pointing. You've got to remember that this was a rally for some deeply stupid people, I'm sure it adds to the security difficulties to have to suss out what's just idiots being idiots and what is useful info.
I think the reality is more simple; people overestimate the abilities of the Secret Service. I am sure they're experts and highly trained, but they're still human, and the mythos around their abilities is likely overblown to an extent. History has shown that if someone is determined enough, they can get close to the president if they want.
They're there to undermine progressives by pretending to be them. It's a way of taking over and deflecting the movement.
An example would be when Shontel Brown beat Nina Turner in Ohio. Brown took money from Republican donors and corporations, yet she got endorsed by "progressive" groups to help her against Turner, who is an actual grass-roots funded progressive.
Same with Mondaire Jones backstabbing Jamaal Bowman (who had defended Jones in the past when he was under attack by the establishment).
This is how Democrats try to co-opt what they can't control, and why it's critically important that voters make it a habit to monitor their candidates funding. The money will always tell the true story.
(Opensecrets.org is a great resource for tracking what groups politicians get their money from)