The 900 lb limit you mention is for an 8.5' long 2x6 loaded in compression, where buckling is the likely failure mode, hence the 900lb rating. Looking at the photos, theirs are only a few feet long. A 4' long unbraced 2x6 is rated at 4247 lbs in the page you linked. Additional to that, the design may also rely on the tensional strength of the member, which is greater still.