I'm going to go out on a limb and say that robbers that have just been locked out would probably act noticeably differently from police or armed citizens trying to render aid.
We can speculate, but at present the only thing we have to render an assessment from is the article and Dosser's quotations in it. He said the men were "super agitated" -- that covers a lot of potential scenarios, to include those mentioned and more. Then he saw the gun and drew his, although he had not yet been detected by them, didn't know what was happening, was not under immediate, direct threat, and was only in proximity to potential threat because he placed himself there.
From the story, it sounded like he was suspicious of their behavior even before he saw the gun, and he did not draw until after he saw it.
More confused; Dosser is quoted as saying "nothing made sense". Then, yes, he saw a gun so pulled his.
Being locked out wouldn't have stopped the robbers from shooting the owner, unless the store had bullet-resistant glass.
No it wouldn't have...yet they didn't, and nothing in the article or from Dosser's description indicates he perceived an immediate mortal threat to a clerk or anyone else.
Again, I wasn't there and am not Dosser -- as stated, I may well have done everything he did. But by the book he did some things wrong. And I think the police department agrees: while understandably lauding Dosser for being a brave, concerned citizen, the police spokesman goes on to tacitly say to the citizens of Minneapolis not to do what Dosser did by reminding readers that concealed carriers are instructed not to intervene in similar situations.
Frankly, in regards to the interaction between Dosser and the robbers, it all played out before anyone knew exactly what was happening and thankfully nobody squeezed triggers.