That's not the point... he might not have had that nickname with anyone else.... however, if Page and Strzok had too many Erics in their various discussions that it was getting confusing, they might be the only ones calling him Charlie.
Yeah but like the Ukraine transcript, there was no expectation that these communications would ever come to light, so why would they be circumspect? “Katie’s husband” isn’t exactly the Enigma Code... I’ll bet they just couldn’t think of his name. Likewise I wouldn’t be at all surprised that Charlie really is his common nickname