I can safely say, from direct (and disappointing) experience that this is true only up to a point. Whereas filters, seals and other minor consumables from Thermaltronics may work well in Metcal systems, I have been very UNimpressed with the "performance" (or lack thereof) of their desoldering tips.
I made the mistake of investing in an entire range, originally intended to sub for Metcal's STDC102 thru STDC106 tips. The ones from Thermaltronics were always extremely slow to heat up (on the order of five minutes once removed from the stand). That, and I found they did not maintain temperature as well as the Metcal tips nor did they recover as quickly.
I finally got tired of being unable to do efficient desoldering and went back to real Metcal tips (though, in all fairness, I do tend to buy new/old stock tips from Ebay to save a few $$). Haven't had a problem since.
So -- Buyer beware! As is often the case, cheaper does not necessarily mean better.