(Edit: I emailed them and the article has been corrected.)

> a Library that contains 251,312,000 volumes of random sequences of letters (1,312,000 being the number of characters in any given book, each of which admits of twenty-five variations)

This seems to be a typo of sorts. I believe the correct number is 25^1312000 (sequence length m, n possible values for each, n^m distinct possible sequences).

Having hit this in the past, my guess is it was originally written with a superscript, then the text lost the formatting (leaving 251312000), and an editor left it as 251,312,000. Always worth checking final rendering for any text you expect to include superscripts.