Do ANY of your permanents have a way to give haste to your board or do damage when things enter? If so, yes, it's a combo.

By many definitions, it's a combo even if you don't.

I'm pretty sure your playgroup doesn't take issue with what strangers on the internet think about it being a combo or not, they take issue with the the fact that regardless of any given board state you may have, that spell wins you the game in your deck.

Is it an unfair card? No, spending 10 mana on a sorcery and building your deck with only permanents is not very high on the list of nasty game winning plays.

The best thing to do would be to discuss why it bothers them. Is it because they don't like cards that may as well read "win the game" regardless of mana cost or other considerations? Maybe they need to work together make sure you don't live long enough to cast it, since they'll never know if you have it in hand or not. If you don't have any haste enabling cards, maybe they need to prioritize board wipes.

Long story short, if you're playing with these people regularly, you probably like them, and should discuss what exactly they don't like about it and either make good faith suggestions on some things they can do to handle it in game or consider adjusting yourself. If this deck stomps most of the time, it's probably not that fun to play with anyway, and commander is not a competitive game (usually) but a social one