I haven't found that exact claim on the site. It does list these blockers as "Liste von bekannten seitenmanipulierenden Add-ons" (known site-manipulating add-ons) [2] and on the main page "Diese seitenmanipulierenden Add-ons stellen ein erhebliches Sicherheitsrisiko für Sie dar!" (these site-manipulating add-ons put you in a situation of increased security risk) [1]. Together, these two assertions might be libellous, but because they are not together, I guess it is not as straightforward as the words of Michael Büker's post suggests.
Ad-blockers protect against attacks coming from compromised ad servers, but also increase your total attack surface. The message is misleading about the relative risks, and it's dishonest in singling out ad-blockers among thousands of extensions, but I wouldn't say it's blatantly false.
Is it libelous against browser makers, though? They spoof the browser's info bar and I believe they style the landing page to look like an internal browser page.
[1]: http://www.browsersicherheit.info
[2]: http://www.browsersicherheit.info/addons.html