First of all, as we learn at the end, the whole diary was made up to lead *Angier* on a wrong track. This means we cannot rely on the fact that he didn't know which knot it was to be the ultimate truth just because it was in his diary (and even less if he just tells him from his own mouth).

So there are different possibilities and I (as well as the movie, I think) cannot give you a definite answer:

1. The statement that he didn't know which knot was a lie and he really knew it. He might have written that into the diary to confuse *Angier* even further or just to not confess his guilt to him, having chosen the wrong knot (or rather the one *Julia* wanted, as denoted by the nod she gives him).

2. Like *iandotkelly* says in his comment, it was indeed always the other twin who told this to *Angier* and he never got told by his brother about which knot it was because that twin didn't want his brother to know it either (maybe for the same reasons of not confessing his guilt).

3. He maybe really didn't know which knot it was in the sense that he suppressed the memory in order to not even confess his possible guilt to himself and not having to face his responsiblity for *Julia*'s death.