Preachers' nonsense about the creation of the word 'Atonement'?

This was not created out of thin air, as is popularly believed, from the idea of 'at one ment'- being made at peace, 'at one with one another'.  Rather, a medieval Latin phrase already existed, adunamentum, which influenced the creation of the English word atonement.  When preachers say this word was created in the Reformation to mean 'at one ment', I have always raised my eyebrows and wondered if this is preachers being careless about fact.  But the reality is that although you could not 'create' a term like that today- the component parts have lost their meaning and the word atonement does not even sound out the crucial part 'one'.  But there was a word, now obsolete, 'to one' meaning to unite (I guess it was pronounced like 'to own').  ie "their marriage is on the rocks but I'll try to 'one' them".

Here's the authoritative note from Oxford Reference:

The word comes (in the early 16th century, denoting unity or reconciliation, especially between God and man), from at one + the suffix -ment, influenced by medieval Latin adunamentum ‘unity’, and earlier onement from an obsolete verb one ‘to unite’.