15 years ago, conspiracy theorists were being asked to prove government spying. It's hard, and the coverup might leave only circumstantial evidence, but it doesn't mean its not true.
Either way, if you're trying to provide a source against the point being made, it needs to be better than that one. Is there any other?
Edit: I did a bit of looking into it.
Tick-borne diseases were definitely studied there, which means general subject was being researched. (a bunch of pubmed studies are listed here [1], and pubmed shows in the author information that they were from Plum Island. Ignore the site if you like, just look at the pubmed articles.)
The geography of the disease also fits a spread from the Plum Island facility. See the same article for a map.
I obviously can't go order and read a physical book off the cuff, but it seems like the Lab 257 book by Carroll is decently reliable, as far as can be expected when making claims counter to an official government position. From wikipedia: "The review in Army Chemical Review concluded 'Lab 257 would be cautiously valuable to someone writing a history of Plum Island'".
Anyway, in summary, there is enough evidence to ask a reasonable question: which puts the onus back on the government, or you, to provide credible evidence that Lyme was not developed there.