Diesels actally do have manifold vacuum, but it is very low. This vacuum is present not only from the resistance the intake plumbing gives, but the air filter restricts the airflow somewhat as well. Conclusively, the dirtier your air filter is, the more intake vacuum will be there. Some diesel trucks have air filter indicaters that work on this vacuum principal. Since many diesels don't run valve seals, a dirty air filter can mean increased oil consumption as well.
Diesels also typically run positive crankcase pressure as well, due to inherent blow-by gas. This gas is vented into the intake manifold where it is burnt by normal combustion.
In other words, the intake manifold has a slight draw, and the crankcase has slight pressure, to vent crankcase gasses.

As was stated before, this guy is hoping to convince you that his product is something special, which it isn't. I bet he'll sell some anyway.

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1991 GMC Sonoma Ext Cab w/Isuzu diesel (converted March Mar 2003) - sold
1994 S10 Ext Cab w/Isuzu diesel (converted Mar 2008)
1998 Toyota Sienna XLE
B6100HST Kubota CUT DIESEL

1994 S10 with Isuzu diesel and 5spd
http://img.photobucket.com/albums/v6...S15/SigPic.jpg