Dramatic license to enhance legend or bolster class warfare.
The Board of Trade inspections included ensuring that passengers had UNHEEDED access to boats and lifesaving equipment.
While there certainly were barriers in the way of gates, ropes and signs warning/restricting passengers from leaving their respective class accommodations, I cannot envision a prison-like atmosphere that full-height gates would impart... nor even a mind-set that would think having such gates installed would be a necessity.
It's not like steerage passengers of the day were thought to be a herd of unprincipled, uncivilized, unlawful hordes that would creep throughout the ship unless sealed in the underworld, under close watch, lock and key...
What... IF such gates existed, why? Were the ship's designers planning ahead for riots or gangs of looters to erupt from the great unwashed steerage class?
There were dozens of places throughout the ship that steerage passengers could slip into other class spaces with hardly any more effort than climbing a rail or following a crewmember thru Scotland Road.
Surely there'd be such gates EVERYWHERE, and certainly evidenced not only OLYMPIC, but other ships of the era...
No... I think modest physical barriers were all what anyone would encounter... and that steerage suffered more casualties from simply not knowing what to do or getting lost while being tended by a tiny contingent of stewards overcome by events.
At any rate, they were plenty other barriers:
They had the barriers of language...
They had no boat drill...
They had no familiarity within the maze of the ship, beyond the few compartments they had chanced to occupy within their class...
They were generally from ranks that followed direction and societal norms...
...and they probably mostly behaved accordingly, waiting for word or permission (whether it was needed or not), believing (as those in 2nd and 1st Class did) there was no real danger until it was too late.
Drama enough, in my book.