quite often this is believed to be true: start-up == entrepreneurship
Well, I disagree here and it looks like you are mixing up 'entrepreneurs' and the "Mittelstand" you mentioned. I don't know if this is generally true, but i'm under the impression that there is some negative sentiment towards 'entrepreneurship' as it is connected with "making money fast" on the internet and/or "just" doing things to make money. That is in contrast to the "Mittelstand" which is seen as creating actual sustainable business aka Old-Economy.
This is obviously NOT true for all 'entrepreneurs' and all "Mittlerstand" businesses.
Sometimes the concepts of start-up, entrepreneurship and "Mittlerstand" get mixed up. I don't think many ppl bother to differentiate those in as much detail as paul graham does ;)