[svat]

It's not a coincidence: in fact, eqn (the ACM paper by Kernighan and Cherry was published in 1975) predates the initial design of TeX (1977), and Knuth explicitly based the math notation in TeX on that of eqn. Initially, like eqn he had no backslashes, but quickly (before any code was written) changed it to one with backslashes.

He discuses these matters of syntax in his Gibbs lecture of of January 1978 (https://projecteuclid.org/euclid.bams/1183544082): see Figure 3 on the top of page 344, where the notation of eqn is marked "Type B" (“developed at Bell Telephone Laboratories”) and the third column is "Type T".