Against the best team in the Test circuit at the time, sure. But that doesn't mean he was the best against the best. I'd take Lillee's matches in the WSC, against the WI in the Test arena and the World XI earlier in his career over Imran's claim that you mention. More proof, more batsmen, even better performances.
It's also disingenuous to compare the WI that Lillee face to the one that Imran succeeded against. Lillee retired in 84 and Imran started to succeed (meaning batter) against the WIndies towards the mid-to-late 80s. Not only did the line-up change, it aged. I'd take the 75 line-up of Fredericks, Greenidge, Rowe, Kallicharan, Richards, Lloyd over the 86 line-up of Greenidge, Haynes, Richardson, Richards, Gomes and Dujon, any day. In fact, if you take Imran's record against WI till when Lillee retired his record is
worse than Lillee - although still fantastic given the side.