Hiya...
I have that kettle, but don’t have the hot rod heat stick (though I was looking into it a few months ago)
At the bottom of the description for the hot rod stick (stand-alone - no element), it states:
“When selecting an element, keep in mind that the Hot Rod's enclosure adds about 2.5 inches of horizontal depth to whatever length the element is”
The 1650mm element is 8.25” long so total will be close to 10.75” - So should squeeze into the 5.5 Gallon kettle as per that review you mentioned - Why don’t you message Bobby (vendor) on this site to get him to confirm?
I have been brewing with this 5.5Gal kettle on a cheap Duxtop induction plate for 12 months.... It works great - But I am about to add an element alongside the induction plate for faster brew days..
I am in Canada, so I ordered a 1650w TC element from Brau Supply and the Brewtech ekettle retrofit kit (Weldless TC fitting, thermowell and holesaws) from OBK.
Also ordered an EZboil and other parts, but that is a different story!!
Good luck!