Hmmm its one of those things IMHO which someone may or may not find offensive.
Jap could be interpreted as short for Japanese and the whole of that thread seems to be thinking along those lines, and briging examples such as Brit for Britain and Scot for Scotland ect.
I have to say I agree with them, I dont find it insulting and nor do I feel it would offend a Japanese person, but on the other hand if you said Japseye than yeah it would be like nigger for the Blacks...but on this occasion I myself see nothing wrong with the word Jap, but I must add that it is on a thin line of offensive content for some people, as its kinda edgeing into words such as 'jappy' 'japseye' etc etc.
Another good example and over the top in the UK is you cant say Paki for a person from Pakistan because apparently its racist which is wank, look at the word Pakistan...pakistan now look at Britain, Britain and scotland, scotland....why is brit and scot not banned? because they aint all on the whole black, and the law now does favour the other races in a white country because of the bullshit in the past...aint fair but it works thatway...so that might be why some interpret jap as offensive such as paki.