When I was growing up I was learnt that the word Allah was just the Arab language word for God
You learned correctly.
OTOH, the mohammedan understanding of "the one deity who created all that is" is so radically different from the Christian understanding of that entity as to be effectively a different being.
When Christian Arabs say "Allah", they're referring to "The Father and The Son and The Holy Spirit" ... a Being whom the mohammedans reject utterly.
When mohammedans say "allah", they're referring to the vindictive, capricious, demonic entity that was gibbering in mohammed's ear ... a being whom Christians pray God will rebuke.
It is. And ‘Allah’ was used by Jews and Christians before Islam. But that’s not the question or the problem.