The Technium process was originally created to allow joining of dissimilar materials and allow designers more flexibility . However, it also economized the manufacturing process, eliminating, the high labour rates of brazers. As steel tubing gets thinner, the temperature has to be more closely controlled. That requires a more skilled and expensive brazer. With 753, you had to certified by Reynolds. Technium allowed frames to be assembled by relatively unskilled labour. The aluminum lugs further reduced costs by eliminating the need to mitre tubes. Using Technium on high end steel was no brainer for Raleigh. The cost savings were sufficent that Raleigh could both undercut their competitors and increase their profit margins.
1993 sounds correct. I have a 1993 product listing showing the RT600. It was listed as Reynolds 531 main tubes bonded to aluminum lugs. The forks was also listed as Reynolds 531. Stay material is not mentioned. The components were Shimano 105SC STI, like your bicycle and the MSRP was $1095 US.
That year, the USA market also got the RT 300 (aluminum w/300EX), RT500 (Easton E-9 Al w/105SC), SP1000 (Ti main tubes, 753 stays, 531 fork w/600 Ultegra), SP2000 (metal matrix main tubes, 753 stays and forks w/Dura-Ace).