I read about that one too.
The problem is, we're talking about the 2015 Broadwell rMBPs here, which use the new SM951-based PCIe SSDs rather than the slower old XP941-based PCIe SSDs in the Haswell Macs.
Sure, the older XP941-based SSDs will fit, but it won't operate at the ludicrous speeds of the current SM951-based ones.