Are you sure they didn't accidentally send you a 27.5 rim?
The wall along the top is frequently fairly tall. The bead diameter of the rim is (on a tubeless rim) the flat shelf where the tire sits. On a hooked, non tubeless rim, the bead diameter is located by the bottom of the hook, minus a couple of millimeters.I only have a thin plastic rim strip since i run tubes. It took both myself and the shop tech along with a metal tire iron just to get one bead on. There was no way the other bead would go on. I just did a measurement on the rim. My understanding is that a 6'er is 559 and a 27.5 is 584. Mine appears to be about 568 or so--weird. I wonder if a 27.5 tire will mount up-- it is a hook bead rim.