Perhaps not all of the Stendker information is marketing. Here in the UK we have a slightly different perspective on the Stendker versus Asian-sourced discus debate.
Firstly, there was a real problem, historically, here in the UK, with Asian-sourced discus being often liable to carry disease. Thankfully, this is much less of a problem, and we now have some ace Asian discus suppliers, but memories do remain...
Secondly, Stendker discus are acclimatised to harder water than most Asian-sourced discus are accustomed to. This is an important issue here in the UK. Some of the less-populated parts of the UK [eg., Scotland, Cornwall/Devon, etc] have very soft water, ideal for Asian-sourced or wild discus. However, much of the highly-populated parts of the UK [most of the SE, London, etc] have very hard water. My tap water, for example, comes off the pure chalk hills of the South Downs. [No, I don't know why these hills are called "Downs"; I walk on them most weeks and they seem to be more ups than downs.]
Thirdly, there has often been a problem with many local fish shops in the UK carrying a few, rather disconsolate-looking discus. [It still happens.] Stendker, by contrast, have sold to the UK through specialist discus suppliers that have kept to high standards.
Consequently, many serious UK discus keepers tend to think carefully about this particular issue. Yes, there may be some element of marketing hyperbole in Stendker's information, but the European versus Asian-sourced discus distinction has some merit still here in the UK...
Of course, whether German-sourced discus will now be more expensive as a result of the wretched Brexit fiasco is another issue. Better not to go there at the moment :-(