In point of fact, Isaac Newton did not "explain just gravity" - he also invented the calculus and developed important insights into the nature of light, among numerous other contributions to science.

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Of course, this might not impact upon the point you are trying to make - you might just have selected a poor example.

However, casting about for a better example (immediately recognisable names who have made a singular contribution to science but did nothing else of note/had no significant, tangential side interests)...

Eliezer was not trying to give examples of people who made singular contributions but did nothing else. Rather, he was trying to give examples of singular contributions that had a lot to say about some things, but nothing of note to say about other things. His example was not Isaac Newton, but rather Newton's theory of gravity.