Reading of Swift's On Poetry correctly
The last two lines rhyme on /ˈaɪtəm/.
The pronunciation traditionally used in English for the Latin word infinitum ends in /aɪtəm/ because /aɪ/ is the English "long i" sound, and vowel letters in Latin words were traditionally pronounced as the English "long vowel" sounds in stressed second-to-last syllables when the vowel letter is followed by only a single consonant.
Similarly, we find /aɪ/ in the pronunciations of sinus, virus, minus as English words.