Metaplot is the continuing history of the setting, which makes it part of the setting by default.Because metaplot has proven that it's definitely bad for TTRPG settings and probably bad for other media.
And metaplot isn't setting. It's sometimes adventures that take place there, but it isn't the setting itself. Metaplot ruins settings. That's not the same thing as being a setting.
I was born in 1976. That was the present at the time, when my campaign (so to speak) began. Now it's 2022, my campaign has been running for 46 years (still on my first character!), and all the events between 1976 and now are history.