Interesting if true. I am dubious about this because I know that congress has taken action about specific drugs before (notably crack) and I would be surprised if they hadn't passed legislation requiring marijuana to be restricted. I don't know how to search for this information, though, or how I would be confident that it didn't exist without being a domain expert.

Edit: Follow-up, wikipedia has an excellent article [1] about the complexities here. Either congress or the executive could easily make medicinal marijuana legal by rescheduling, but to legalize for recreational use, they would have to amend an international treaty [2], which would require congress to act. Apparently congress has not explicitly scheduled marijuana (as it has other drugs), so the executive is free to act within the constraints of the Single Convention.

[1] https://en.wikipedia.org/wiki/Removal_of_cannabis_from_Sched...

[2] https://en.wikipedia.org/wiki/Single_Convention_on_Narcotic_...