Uh...
Apparently you're on drugs. Yes this is a very real thing. You actually CAN equip legendary items starting at level 101 as well as equip any BoE epic 101 items that can titanforge up to the max of 955. 1 million dps on a lvl 101 character is not impossible because you can literally be better geared than some 110s with BoEs. Not only that but the stat scaling means that a level 830 piece of gear on your 101 will be significantly better than on a 110. You still get 1300 haste rating per item but 1300 haste on a 101 is something like 5% and on a 110 is like 1%
They were definitely just running through the instance by themselves and killing everything in one shot. Also dps scales based on the level so a level 109 will see level 101s doing significantly more damage than is normally possible because of the way scaling works. If the level 101 hits for 500k at their level and that one shots the mobs in the dungeon but for you the mobs have 6 million health then that level 101 will appear to have just done 6 million damage in one attack. It can definitely happen.