Fleetfeather Sandals does have the flying keyword, but it's ability is to specifically give it to an equipped creature. If it gains life through [[March of the Machines]], it won't have flying / haste on it's own. That's the difference between it and Smuggler's Copter.

The Kaalia player was not baiting for information. In your Slaughter Pact / Magnetic theft situations, they both rely on hidden information, something the other player won't ever know until it happens, so not at all like the current situation.

In the Copter / Kaalia debate, I'm really on the fence about it. I see why his opponent would be upset. The question about having any fliers comes up a lot at my table, and we use it to make the game go faster, and not everyone can remember what's going on with 3 other players at the table. It can be a drag for each player to recheck the board state every turn, and EDH already takes up a lot of time.

I think it's really up to how casual the group is. My play table is much more on the casual side, so that skews how I'm looking at things here.

Is it a dick move to not mention Copter?

In a more casual setting, yeah, probably. In a competitive setting? No, it's not.