If you're serious about protecting your head and are going fast, then don't skimp. Snowboard helmets are OK, but they're made for very different conditions than the street. The ebike helmet you linked to looks pretty much the size of a half helmet for a motorcycle. The weight isn't listed in your link (or I can't read enough German), but a Burton snowboard helmet weighs 500-700 grams. A DOT approved moto half helmet of the same look, like a Vega XTS or Fulmet Hombre, weighs a little under 1000 grams. My AFX motocross helmet (DOT approved for street) gives you real chin protection and lots of open air, but only runs 1250 grams. Its in no way "heavy", the protection is far superior to a bicycle helmet and they cost much less than $100.
Also, wouldn't that German ebike helmet be rated for less than 20mph since there's a EU ebike standard top speed.
Also, wouldn't that German ebike helmet be rated for less than 20mph since there's a EU ebike standard top speed.