Yet, some missions require us to play solo.

"Multiplayer" doesn't have that meaning, it simply describes number of participants it doesn't refer to activity. Thats a subset ie solo, co-op and team. If you had zero interaction with other players but could see them in the sandbox, its still multiplayer. Destiny was designed to support all 3 as most mmo do, more demographics = "mo money". If Bungie released play data I'd bet hard cash 70% + of play time is done solo.

"one of"