As mentioned in the comments by F1Krazy already, this information is likely wrong.
The nickname actually originates from his First-name, Shigeo. The Kanji for which can alternatively be read as "Mob" (モブ, Mobu).
This is also confirmed to be the source of his nickname in the Character guidebook as mentioned on the Mob Psycho fandom, a copy of which is included at the bottom of the post.
All though Mobu might not be wasei-eigo for "background character", it does infer such a character. Some more background on this can be found on Japanese.SE's question: Etymology of モブ