Native SAE-speaker here, studied linguistics in undergrad, and "least common denominator" is a perfectly normal construction.

The web isn't a perfect corpus, and Google isn't a perfect corpus analysis tool, but with those caveats...

Google results for "least common denominator": 738,000 / Google results for "most common denominator": 656,000

Additionally, as you point out, Wikipedia lists both as synonyms.

Your original comment was correct and idiomatic English.