No, I don't think it was. I think that's the auto-calc at work. I believe what the game does is first check if you possess a valid method of assault: spy who opened the gate, artillery capable of breaching a gate or a wall, or constructed siege equipment. If you do, it lets you assault. Only then, if you choose to fight the battle on the map does it worry about the difference between infantry and cavalry. I've noticed that it only places as much equipment on the map as you have infantry units. You can still construct siege towers, ladders and rams with an all-cavalry besieging army. It's just that when you get to the battlemap it won't be there. I've done that a few times and hired mercenaries on the final turn to make the assault.

However, I've also noticed that the autocalc results are way off what they would be in a real assault when cavalry are involved. I can autocalc a battle and win with minimal losses where I'd be slaughtered trying to take the city, just because of the calculations. I think what happened is he constructed the gear, the game let him assault because he had the gear, and the raw strength of his army outclassed the defenders and therefore he took the city.

Want to confirm it? Try it with an all-cav army. Lay siege, construct rams, and don't fight on the map, just use the auto-calculate function. I bet it works for you too.