I agree with you generally that it can be a matter of lack of resources/time but in this case, the game has been in development for how long? The current release date is definitely locked in stone but the publisher has seemed to be ok with delays up until now.
I wouldn't assume it's the devs; it could be the result of management monitoring the time/money equation.
They might be able to tweak the engine to run better on the Series S but would that make the game late? Would that come at the expense of other features that are cross platform? Everyone is quick to blame the developers themselves but they get their marching orders from project managers, who get deadlines and budgets from managers above them.
It's impossible to know from the outside where the failure is and speculation is about as useless as pretending any of us would have bought this game for full price at any frame rate.
If it was a matter of a lack of time though, why not mention a plan for a 60 FPS patch post launch? Is support for the game going to be dropped almost immediately?
I agree that it's not possible to 100% know from the outside but they're not helping themselves by not flatly giving an explanation.
Don't forget, they made this admission only a week before launch on Discord. Does not help their appearances.