I don't think they say Oswald said he was alone.
FBI agent James Bookhout, wrote:
"OSWALD stated that on November 22, 1963, he had eaten lunch in the lunch room at the Texas School Book Depository, alone, but recalled possibly two Negro employees walking through the room during this period. He stated possibly one of these employees was called ‘Junior’ and the other was a short individual whose name he could not recall but whom he would be able to recognize." (WR, p.622)
Secret Service agent Thomas Kelley wrote:
"He said he ate his lunch with the colored boys who worked with him. He described one of them as ‘Junior’, a colored boy, and the other was a little short negro boy." (WR, p.626)
What Fritz wrote: "say two negr. came in. One Jr. + short negro" refers to what Bookhout and Kelley wrote. There's nothing in all these accounts about Jarman and Norman entering the building through the back door.
And so, we have three different versions of what Oswald is supposed to have said.
We can be pretty sure, however, that Oswald, who knew the building and first floor lay out better that any of the three men, would not have said that "Junior and the short negro boy" walked through the lunchroom as that is simply impossible. Bookhout also doesn't say they walked through the lunchroom. It is however possible, and indeed more likely, that he saw Jarman and Norman come in, through the back door and walk towards the elevators which Bookhout describes as ""walk through the room".
There's nothing in all these accounts about Jarman and Norman entering the building through the back door.There doesn't have to be. Where else would they have entered the building, on their way to the elevators, if not through the back door?
A fact that can't be denied is that all three reports confirm that Oswald said he saw "Junior and the short negro boy" (who we now know to be Jarman and Norman) when he was in the lunchroom eating his lunch. We know from the testimony of Jarman and Norman this could only have happened minutes prior to the shooting.