To be honest, I'm highly doubting this claim entirely, they'd have to do as Zemen said and base it right before the events of MM, however, that wouldn't make sense as there is not enough time to do so. From the starting clip to Majora's Mask we can see that Link didn't even know about Termina before arriving there, and from that point in the game, he practically arrives at the point in which he starts his quest to retrieve the Marjora's Mask for the Happy Mask Salesmen, there is barely any time in which Link could go off and have an adventure, no matter how quick or small Nintendo tried to make it.
As far as I remember Link ends up in Termina and chases the Skull Kid because he stole his Ocarina of Time (not sure if he stole anything else) and shortly after Link ends up in his Deku like form, from then on in the game you spend the time chasing Skull Kid with one of his faries (can't remember which, it's either Tatl or Tael anyway :xd
and ends up having the discussion I've spoken about earlier, the time in which he talks to the Happy Mask Salesmen who asks him to help him retrieve the mask before 3 days passes (although I'd like to know why he happens to be leaving the exact day that the Moon hits Termina...hmm).
However, there is a section between the end of OoT when Zelda sends Link back in time and the point at which Link ends up in Termina which has yet to be fully accounted for, yes, we are told that he was searching for Navi but we haven't been told how long for in comparison to his little adventure in OoT (well, gargantuan adventure to be honest :xd
....so I guess there could be a rather quick and shady plot stuck together in this little time space. I highly doubt this though as I believe in the theory that Link finds that Master Sword in Zelda Wii and if the game were to be placed before MM and after OoT that just wouldn't make sense, considering Link had already known the location of the Master Sword and actually aquired it before being sent back in time.