Baldur's Gate 1 wasn't "close to non linearity." Baldur's Gate 1 was not linear, at all.
I think it was a relative statement. It was more nonlinear than BG2, NWN, and KOTOR. In a sense it was more widopen because you had the freedom to go to alot of places on the map because of the wilderness areas as opposed to what BG2, NWN and KOTOR did where you can only go places that the game has opened up for you. In all three of those actually, you're kind of forced to go along the main quest just to open up more areas to explore.
I don't see how it could be perceived as relative.
He says its "the only game that bioware has done which is anywhere close to non linearity."
Its not "close" to non linearity. Its not linear, at all. You are almost never forced to do anything (barring the escape from candlekeep both times), and even when you are in the linear events, they're short, and you're not forced into them.