The free bundle was first spotted and posted on Reddit without an official comment from Microsoft, causing some confusion over whether the game was now free, or whether this was simply a weekend promotion. Xbox Support later chimed in on Twitter, noting that the collection is indeed free-to-play for this weekend only.
In any case, if you want to give the Handsome Collection a shot, you'll get to try out both Borderlands 2 and Borderlands: The Pre-Sequel. Do note, however, that the free weekend bundle is located in a separate listing as the Borderlands: The Handsome Collection Unlock Bundle. The game's usual listing is priced at $24.99.