The record shows that LHO was a compulsive liar. He lied about many items that could easily be checked. Even when there was no apparent reason to lie. In my opinion, this was just another fabrication and has no significant meaning to me.
This claim that Oswald was told by Shelley that it was okay to leave and that's why he did so is simply, for me, not supportable. Apparently the source for this is this FBI report (neither Fritz nor Hosty mention this Oswald/Shelley conversation):
Full source/link:
https://aarclibrary.org/publib/jfk/wc/wr/pdf/WR_A11_ReportsDPD.pdf pg. 619
So this account says that in Oswald's opinion that based on Shelley's remarks no more work would be done. Nothing about Shelley actually saying this, nothing about Shelley saying it was okay to leave. Furthermore, if Oswald is out there with Shelley then he, Oswald, can simply ask if it's okay to leave. But he didn't. He just took off. And no other worker took off at that time.
Added: Oswald can say here that he's not lying. He thought, based on Shelley's remarks, that because no more work would be done he could leave. The fact that Shelley was in the next room wouldn't prevent him from making this claim. Again, he apparently said it was just his opinion.
Unless there's another source that I'm missing this idea that he left based on Shelley's explicit permission is not supportable.