That's not the part I'm decrying as unrealistic -- yes, it makes sense to loan him an available X-Wing because they're throwing everything they have at the Death Star and they have more starfighters than pilots at that moment. What's unrealistic is Luke being a "hero" pilot who outflies everyone at everything.
Luke has the highest performance of any Rebel pilot at Yavin, in absolutely all tactical roles -- it's not just using the Force in the trench run, he also demonstrates superior skills in dogfighting TIEs and strafing the Death Star.
No, he's not. He's one of only two Rebel pilots who gets a TIE FIghter kill onscreen, with Wedge being the other one, and also one of only two Rebel pilots who is able to evade a TIE Fighter long enough for a wingman to shoot it off him. He is also more effective at strafing the Death Star than any other Rebel pilot, being the only one who scores penetrating hits that cause substantial secondary explosions.
Keep in mind, the rest of the thirty Rebel starfighters were wiped out in the battle over the Death Star by the remaining four TIEs, with only one Y-Wing, two X-Wings (Luke and Wedge), and the Millennium Falcon surviving the total battle. This demonstrates just how superior Luke's offensive and defensive skills were -- he was performing at highly skilled ace level, on par with Wedge and Biggs, and vastly superior to all of the unnamed Rebels.
As for Luke's supposed flying experience on Tatooine, he's flying a freaking crop duster. At best, he's flying the equivalent of a Grumman Ag Cat and then jumping into the cockpit of a Hellcat without so much as a check flight.
It's useful to recognize and be able to deflect this kind of pedantry in debates so I want to point out what's happening.
Someone is
arbitrarily limiting the field of discussion to a single narrow section in order to make a point that's easily disproven if you look at the bigger picture. You'll see similar tactics where, for instance, people will emphasize that Rey is the first female main character in Star Wars and then carefully limit it to only
main-series movies to eliminate the much-more beloved Jyn Erso and Ahsoka Tano, because "Star Wars fans hate women" falls flat if readers ask why there's a beloved female main character in a movie released just before Rey's debut, or start to wonder why the single most popular character in all the animated series' is a girl.
Specifically this argument is based around limiting discussion to just the Death Star attack to paint Luke as a Stu.
Throughout the movie multiple scenes say Luke is a great pilot. But...
He gets socially dominated by his uncle.
Tricked by R2D2.
Has his ass handed to him by Sandpeople (saved by Obi Wan)
Saved by Obi Wan
again in the bar.
Can't do anything but watch (and get put in his place by Han) during the escape from Tatooine.
Has to be saved by Wedge in the Death Star attack.
Nearly gets killed by his own fireball when he flies too close in the Death Star attack.
Leia winds up having to come up with the plan for her own escape when Luke's plan fails (not entirely Luke's fault given Han's acting but still Luke's stupid plan),
Has to get saved from the Trash Compactor by R2-D2.
Saved
again by Obi Wan's sacrifice.
Manages to tie Han for kills against the TIE fighters after the Death Star but hardly dominates, and Han gets the first kill.
Has to be saved by Wedge during the Death Star attack.
Nearly dies in his own attack on the Death Star because he didn't know to pull up and avoid the fireball when he shot too close.
Has to be saved from Vader by Han.
Luke is referred to as a great pilot in multiple points through the movie, carefully laying the groundwork for the final stage. In the last action sequence of the movie, the scene where your standard issue hero gets to show off how they've grown, he
finally gets to pilot for the first time and show off his stuff which has been foreshadowed and alluded to repeatedly as a Chekov's Gun throughout the film, and even then does he take the big bad down? Nope, Han Solo saves him and Vader escapes.
Luke has a great many failures and scenes where he needs others and has to be rescued, which are generally Anti-Mary Sue markers. You have to skip those scenes to call him a Sue successfully. Arbitrarily limiting it just the last part with the Death Star trench run, which is basically Luke's
only scene to shine, is like taking a Rocky movie and saying Rocky's ridiculously overpowered, because if you look at the last 30 seconds of the last fight he just plain beats his opponent down in a flurry of awesomely powerful punches. You need to limit the discussion in order to ignore the other 119.5 minutes of movie that are all about Rocky Balboa's endless struggles both in and out of the ring.