Which is what you are supposed to do when the rules are silent on something. I'm not criticizing your on the fly ruling or the outcome. I do want to draw attention to the fact that it happened only because you allowed it to happen, and that is not a semantic difference.I think that’s just arguing semantics. Sure, I enjoyed the paladin trying to pull something like that off. To me, that’s the sort of sacrifice that a classic paladin would make, when hope is nearly lost. I adjudicated an opposed role and the villain failed their check.
On things where the rules are silent, neither you nor the player really have an understanding of whether something is possible and if possible how likely it is. And the difference between a ruling and a rule is that they are situationally applied according to what the DM wants to have happen - in this case the sort of sacrifice a classic paladin would make.
You adjudicated an opposed roll as the resolution method of grabbing a villain and tackling him out of the window. Great. It makes sense and it makes for a great scene. But there is more than semantics going on here.
If a group of zombies or goblins are in the same room, and they now attempt to throw the party out of the window, do they get the same ruling as the Paladin? One opposed roll and out the window you go? Would that have made a good scene? Do you similarly play opponents as clever and devious against the PC's? Or take it up a notch. Suppose a party caster sees this opposed roll ruling and thinks, "That's a real bargain.", and the next time you have a scene like this they use some sort of summoning spell to conjure creatures that grab things and defenestrate them without the need for a PC to die. Does that make a good scene? Do you still make the same ruling or do you adapt by never again putting NPCs in situations where they can be tossed down shafts in throne rooms by Paladins (fallen or otherwise) in acts of sacrifice?
Most of all, do you think this ruling is going to change the way your game is played? Is this ruling now a house rule, or are you going to walk it back in situations where it doesn't make a great scene?
As a 1e AD&D DM, I was well aware that per the official rules, low HD mooks were better off attacking the PC's with grapples and punches than they were with swords. Unarmed combat was far deadlier than armed combat for all but the most effective armed combatants. This made me both reluctant to employ it and reluctant to validate it as a player tactic. What I wanted was rules that made it situationally great - like when you needed to defenestrate the bad guy when all hope was lost - but which didn't make it the go to tactic by everyone in every combat. And, figuring out how to do that took some heavy lifting. Like, for example, I realized I needed to define what 3.0e later defined as an 'attack of opportunity' - I called it at the time somewhat confusingly a 'parry' and it had almost the exact same rules 3e would later come up with (only you didn't need to take a Combat Reflexes feat, it was built in, because I didn't have feats) - so that those goblins would hesitate in their swarming and defenestrating or simply just tackling the player to the floor. Then, both the players and myself could decide to make these propositions when the situation suited it, and not just when we thought that it should be validated as a great scene, and I wouldn't be stuck trying to decide in the middle of a game if I was being fair to the player by allowing or disallowing something or playing the monster fairly by allowing or disallowing something or exactly what ruling I should make.
One take on this you might have is none of this really matters. You can just make rulings situationally - Paladins get opposed roles on BBEG's because they are big dang heroes, and goblins who are not just don't do that sort of thing. And that is fine, but then I think you still have to admit that even if that is your preference, it's not just a semantic difference.