intended target and possible target are different things.
The law, as it exists, targets any and all communities with “a significant number of United Kingdom users”. There is no minimum size requirement.
It might be possible that the law turns a blind eye on smaller communities. But it might not. They are in their right to go against a 20 user fediverse instance the same way as they are against facebook
My instance seems to do that for me. Making this post look very confusing for a moment