From the USOC to Governor Baker:
http://www.sunherald.com/2015/07/24/6335359/apnewsbreak-usoc-asks-mass-gov.html
Another un-sourced quote, great. So this might play out to be BS.
However, if the rumor gets confirmed, Baker should tell them to take a flying fuck at a rolling donut (on the deadline, not the whole bid).
Boston 2024 and the USOC dropped a severely flawed plan into Baker's lap on his inauguration day, and Boston 2024 has been flailing all over the place since then. Bid Version 2.0 was released June 29 and Boston 2024 has been publicly declaring "never mind that version 1.0" in their attempts to not release the full Version 1.0. And Bid 2.0 still has some pretty huge holes in it. So Baker's team has had less than one month to review the "real" bid, which is still a significantly incomplete plan.
The USOC can set whatever deadline they want on the Governor, but the only way it makes sense for them to do so in this way is if they have lost patience with Boston 2024, want to move support to LA, and are trying to find a weasel's way to shift the blame to Baker and still have the time before Sept 15 to scramble together a bid with LA. The article linked above suggests that's the likely reason, though it does not attribute it to the un-named source.
It's not Baker's fault that the version 1.0 was so flawed, nor is it Baker's fault that the USOC so utterly failed to see those flaws, nor is it Baker's fault that the USOC may feel backed into a timing corner on the September 15 date by which they put in the name to IOC.