On the info page:

""" <div><span class="highlight">M</span></div>

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<div><spam class="highlight">M</spam></div>

<div><span class="highlight">M</span></div>

proportional fonts can make it easier to spot typos """

The example given demonstrates that a certain subset of typos can be caught using proportional fonts, but not all.

If a word has too many or too few characters in it, I think you are more likely to spot it utilising a monospaced font.

EDIT: the quote has been made more legible.