Why? Because no one can define ‘micro-transaction’ fully. It’s literal definition would be ‘a small payment made in exchange for a piece of extra game content’. As such, every DLC, even expansion pack for every game can be defined as a micro-transaction.

In the same conversation, the same developer says “there will be DLC, which you will obviously pay for” – what does this mean? Does it mean extra missions? Areas? Or does this mean ‘weapon packs’ and ‘cosmetic sets’ like it does in other games?

The huge ambiguity in the term micro-transaction, as well as DLC, gives a LOT of wiggle room.