Luke Skywalker taking a lucky shot at a vulnerability that a team of engineers and military men, all of which were high-level Imperial defectors, with support from many planets of what is the Star Wars alternative of Western Europe and North America, had found by analyzing space station’s stolen blueprints, using computers and what not, is realistic.

I'm guessing you haven't seen Rogue One. The architect of the death star was sympathetic to the rebellion and deliberately created the vulnerability of the reactor that needs only a single hit with a blaster to blow up the entire megastructure, sent a message to the rebellion explaining said flaw and instructing them to aquire the designs of the death star to identify where the reactor is so that they can exploit the flaw.

Having been involved in large (software) projects this seems quite plausible that someone near the top could intentionally leave a backdoor in there and have it go unnoticed into live testing, especially with the mix of disciplines needed in constructing such a megastructure