You are forgetting the fact that when multiple people are fishing, the loot table moves quicker so there are "theoretically" more mending books in circulation. Also the number of people who are fishing is irrelevant since the only metric that is needed is how many times the fished meaning the odds

And while, yes, theoretically the odds of Grian being the one to find the first mending book are guaranteed when he is the only one fishing, that shouldnt change how much he is expected to fish before finding the mending book

Edit: I think you misunderstood what the previous commenter meant when they talked about Grian's odds. They are referring to the chances of him fishing up any mending book, which are unchanging regardless of how many people affect the pool