The original Matthew might have been the Matthias chosen to sub for Judas who busted his gut, but whether this is right or not the idea goes back to Schleiermacher and the other biblical NT scholars of the 1800s. The original circle of 120 who met in the Upper Room were mostly Jews so most were literate and were keeping notes so Luke could compile his accurate history in two books. They all would have had access to this hypothetical original Matthew in Hebrew.
It sounds all very speculative. I am skeptical of the idea that Matthew didn’t write Matthew in the absence of any definitive proof. Scholars often try to make their bones by these sorts of theories. Shakespeare is an example where they try to credit his work to others. This is what a friend of mine calls “a pipe and beer question”-fun to ponder and talk about but pretty fruitless in terms of increasing knowledge.
I am also skeptical about Matthew being written in Hebrew. Koine Greek or aramaic is more likely. Do you know of any early versions found that were in Hebrew?