It seems that Will Fritz wasn't in a lying mood that day. He wrote down that Oswald was "out with Bill Shelley in front" and that was the most damning thing he could write down. Fritz certainly didn't make it up. He wrote it down because Oswald said it. And likewise, he wrote down "home by bus" because Oswald said it.
It means that Oswald definitely said that he went home by bus. So, if he didn't do it, why did he say it? Why would he lie about that? Whoever supposedly picked him up wasn't a getaway driver because Oswald hadn't done anything. It may have been against the Depository rules for him to leave, but it wasn't against the law. There were no criminal issues involved. So, why would Oswald have lied about it?