I wonder where you get this estimate? I mean, are you privy to their total unit sales per year, including OEM stuff? I doubt it.
Seriously, think about it...
They've done manufacturing for other folks for decades. They have had contracts with MAJOR players in the audio market. The only people we know of who are buying them online (as TC Sounds, not as some other brand) are the ones essentially on the various car audio forums... so I'd say prolly a few hundred people to a few thousand at absolute max. Heck, if you looked at their forums, they weren't even all that busy.
I'm not saying your totally wrong, as I don't know, but I seriously doubt you've got the background info to spout off numbers like that. I dunno, though, maybe you do.
Care to share why you hold that opinion?