Microsoft has earlier announced that they will release the Insider Builds to Lumias which were shipped with Windows 10. With the Introduction of Redstone Build now they had made that true.
From Windows Blog:
“As we mentioned a couple of weeks ago, initially we’ll make new builds from our Development Branch available only to devices that shipped with Windows 10 Mobile. Currently this includes the Lumia 950, 950 XL, 650, 550, and Xiaomi Mi4. We plan to expand from there as we release Windows 10 Mobile to other devices.
Soon Redstone Build will be available to other devices also.