John Ayto, Dictionary of Word Origins (1990), notes instances in English long before World War II in which holocaust referred not merely to "a huge inferno" but to a slaughter:
Etymologically, a holocaust is a 'complete burning,' and the word was originally used in English for a 'burnt offering,' a 'sacrifice completely consumed by fire' (Mark 12, 33, 'more than all whole burnt offerings and sacrifices' in the Authorized Version was translated by William Tindale in 1526 as 'a greater thing than all holocausts and sacrifices'). ... John Milton was the first English writer to use the word in the wider sense 'complete destruction by fire,' in the late 17th century, and in the succeeding centuries several precedents were set for the modern application to 'nuclear destruction' and 'mass murder' — Bishop Ken, for instance, wrote in 1711 'Should general Flame this World consume ... An Holocaust for Fontal Sin,' and Leitch Rithie in Wanderings by the Loire 1833 refers to Louis VII making 'a holocaust of thirteen hundred persons in a church.'
For a sense of how surprisingly slowly the shoah sense of holocaust came to dominate all other meanings of the term in the post-World War II era, consider this complete entry in Evans and Evans, A Dictionary of Contemporary American Usage (1957):
holocaust; disaster. A holocaust is a Greek word meaning something which is burnt whole. It was used for a burnt offering and has come to mean, especially in journalistic writing, a great or wholesale destruction of life, especially by burning (Incendiary bombs led to a holocaust in the slum quarter). Holocaust is often used as a synonym for disaster, but there is a difference: a holocaust may be a disaster, but there are many kinds of disasters which are not holocausts. Disaster (which means literally a bad configuration of the stars, i.e., just bad luck) designates any unfortunate event, especially a sudden and great misfortune. A holocaust may be accidental, but it may also be the result of human intention. A flood, a railway wreck, or the collapse of a building may be a disaster, but none of these things is a holocaust.
And here is a second (and even later) complete example, from Morris and Morris, Dictionary of Word and Phrase Origins (1962):
holocaust Holocaust (pronounced HOL-oh-kost) is a word we sometimes see in the headlines, yet it is not one that many of us find in our speaking vocabularies. Originally a holocaust was a sacrificial burnt offering to pagan gods in pre-Christian times. It is derived from the Greek words holos (whole) and kaustos (burnt). Nowadays it is generally used to mean slaughter and destruction on a very wide scale, especially by fire, as in the sentence: "All London was a holocaust after the bombers left."
It is unimaginable that a serious reference work written today would discuss the term holocaust without specifically mentioning the Nazi-orchestrated shoah at some point.