The article only mentions Muonium in passing -- I guess it's only somewhat exotic? -- but I can assure you that it's very real. Chemically it acts as a light isotope of hydrogen, and the interaction of the (anti)muon spin makes it useful for probing reactions in otherwise problematic environments -- for example, supercritical water, where the pressure chamber needed for hundreds of atmospheres of pressure gets in the way of most sensors.

> Are those proposed, or already predicted composite atoms, or have they been already found?

Not an expert, but following the Wikipedia links, some of them are real...

Positronium was "experimentally discovered by Martin Deutsch at MIT in 1951" [0]

Hypernucleus was "was discovered by Marian Danysz and Jerzy Pniewski in 1952 using the nuclear emulsion technique" [1]

[0] https://en.wikipedia.org/wiki/Positronium

[1] https://en.wikipedia.org/wiki/Hypernucleus