There was absolutely positively 100% no railroading that occurred, and anyone who thinks otherwise, I apologize, but you are just flat out wrong.

A Dungeon Master sets a stage. As Danny Pudi put it in Community, 'I create a boundless world, and I bind it by rules'. Matt has set the stage for 50 episodes. Bell's Hells was up against incredibly large odds, and they knew that going in. They made the best preparations they could, and took their shot. It worked-sort of, as they seem(?) To have increased their timetable by destroying the power sources.

Where did Matt railroad them? By putting them up against a stronger foe? What is more likely, that a group of adventurers stumble across monsters that scale along with their level by chance? Or that sometimes, you would come up against threats you are unequipped to handle? My thought is the latter.

'Telling a story' is not railroading, and it never has been. As OP said, the dice rolls were just not there for the sake of the story. Two failed persuasion checks against Lilliana, Chetney's failed attack on Ludinus, Fearne's failed arcana to notice the dodecahedron, FCG's failed Dispel Magic on Caleb's collar. There are multiple ways this story pans out- just because we saw only one, don't assume that's the ONLY one.

I urge everyone calling this railroading to take a step back and look at a larger picture. A villain had time to plan these things for 1,000 years. He attacked the Ashari JUST TO PUT KEYLETH ON THE TRAIL TO LURE VAX OUT. Is it railroading that he had a 5th level spellslot to cast Wall of Force to stop the Silver Sun? How about the fact that, without saving throws, the crash killed 95% of the lackeys who would have made moving about her difficult? That sounds to me like the decisions the characters made mattered. They didn't stop the big bad on episode 50 at level 9, no. That does not equate to railroading.