Only if it gets confusing. Context is, as you say, usually good enough.
Sure. I think many sentences could have much more clarity if most instances of ‘metagame’ were replaced by ‘game’, though.
But let’s really go for the throat and ponder - how big is a meta, really? For instance, the UK Nationals has over a hundred playing the game. But that metagame is a very strange product of disconnected regions of the UK (where players and their biases produce different equilibriums to other regions). So I’d say the metagame of Nationals is certainly ‘larger’ than the number of Nationals players alone, but it’s not obviously just the sum of all players the participants of regularly played with, either. Similarly, many players play online. How do we ‘count’ that gobal contribution?
So how ‘big’ is the metagame of an event? I’m really not sure how to assess that. But it’s probably not best expressed as just the sum of the people directly involved in the event(s) (and particularly not for rare or one-off events, which are likely much more shaped by factors external to the event than some equilibrium of the participants, as there’s no time for such an equilibrium to be established).
Conversely, questions like “How many people do you get in your meat space?” is much more well defined, but maybe not as interesting. So I’d argue there are two different questions here, and two different terms would be useful to avoid hiding that.