it's probably stylistic - i don't know whether the correspondents would have written that or not but it may well be the way they present them in print. so it sort of doesn't matter whether the letter-writer knows or assumes male or female editor, the change is probably made anyway
private eye do the same so it comes across as mocking a rather pompous style
i had a quick look at the online letters to the london evening standard (the only paper with a female EIC that i can think of) and that doesn't start with any sort of address, it just launches into the letter