Well, Rotel uses Dolby and DTS, and prior to Dolby Pro Logic IIx they also used a Rotel-branded version of Cirrus's surround back processing (very similar to the "Cirrus Extra Surround" offered on the Model 950), but otherwise I think the explanation you were given is pretty accurate.

Here's what I think it boils down to, for what it's worth. THX has provided some valuable specifications and guidance to the home theater industry. Not only did THX help identify good hardware in home theater's earlier days, but THX helped identify design issues that needed to be addressed in all home theater equipment, thereby benefiting all of us in at least some form. Even today they can be useful, both in a review role for some very impressive THX-approved hardware and in helping keep pressure on the industry to maintain high standards. For those reasons, dismissing them outright is unreasonable, even if their marketing program's growth (computer speakers, Monster speaker wire) may have diluted some of the perceived prestige of the brand. At the same time, classifying THX as a core feature or equating it to Dolby or DTS is also unreasonable. DVD owners who want surround sound need Dolby Digital and DTS, because those are the data formats in which digital audio is stored on DVD's, whereas there is no THX audio format that requires THX-listed hardware to decode. There is a wealth of excellent quality hardware on the market that lacks the THX logo - often that hardware represents a particularly good value for the consumer in part because the manufacturer elected not to pursue THX licensing, thereby keeping costs down and offering a little extra savings for the consumer. I think at least some of the seemingly anti-THX activity online stems from people who don't want newcomers to the hobby to deprive themselves of the opportunity to discover some of those great values due to an over-emphasis on the THX logo.