Their right hand side doesn't mention it, so I'm intrigued whether this sub-reddit pre-dates Language Log naming almost this same type of mistake an eggcorn?

Specifically an eggcorn involves an erroneous analysis, in the name example a person figures an acorn does look rather like an egg and so maybe that's why the name for it is "eggcorn". Likewise "tow the line" mistakes the metaphor as involving pulling a rope rather than standing against a chalk mark ("toe the line") and "free reign" assumes it's about some analogy to absolute power of kings rather than controlling horses.