As you can see, he had been reading Salomon Maimon's Lebensgeschichte (Autobiography; Berlin 1792). Maimon, who among other things wrote a commentary to the Rambam's Moreh Nevukhim called Giv'eat Ha-moreh, had assumed the surname Maimon as a tribute to Maimonides. As you probably surmised, there weren't too many Maimons in 18th century Lithuania (or Berlin). Amusingly, Mackintosh is not entirely convinced that Maimonides is a modern philosopher, although of course he has no idea if Maimon is portraying him accurately, not knowing the Guide for the Perplexed at all.
Two days earlier Mackintosh had written that "the manners and conditions of the Polish Jews are quite new to me. I never before caught a glimpse of that modification of human nature." Overall, he decides that the book is "a most entertaining piece of self-biography."