So while Boris's observation is accurate, it doesn't rule out later corruption of Agatha and/or later infection of Gil or Klaus.

However, Klaus claimed to have evidence that Gil had been infected while in Paris, before he ever met Agatha, and he'd (K.) previously told DuPree that Rovainen thought Agatha was the Other--that he (R.) was compelled to obey her. Presumably Boris had already heard this from Klaus, which was why he (B.) went to Selnikov for answers. That is, even if Gil had been wasped later, Klaus would still have been wrong or lying about when it happened.