Because it makes the story of not seeing Tanner much more credible silly.
And comforting you also recognise that Gerry's arrogance knows no bounds, even if by failing to reign it in undermines his prime witness.
Ah yes, I forgot that element of the theory - Gerry needs to undermine JT's credibility as a witness and alibi just in case she was spotted by someone on a balcony whilst walking past when the two men were not actually there - or something like that, isn't that right Faithlilly?
So to recap - GM needs everyone to believe JT saw the abductor at the same time as she saw him, but he also needs to undermine her sighting by asserting that he never saw her at the time she claims to have walked by. You see how sensible and logical this theory is? Perfick!
PS: Why do you find it comforting that I have mentioned Gerry's stubbornness and refusal to accept he might be wrong, whilst at the same time refusing to acknowledge that this is a probable explanation for his insistence about where he was standing that night...?