No, it was not all of them.
Looking back at my notes, it seems that I checked three, on different days. Each time I compared with one or two skulls from other breeds (not always the same other breed, but always not-crested.)
One Spitzhauben skull seemed completely normal.
One appeared to have a slight raised area (not big enough to be a vault), but no holes or weak spots.
The third also had the small raised area, again not big enough to be a vault. There did not seem to be any actual holes, but that skull did have a weak spot: when I poked around with the point of a knife, the knife went in with a crunch, which did not happen with the normal skull I was comparing it to. That particular bird did have a larger crest than the other two (larger than a Spitzhauben is supposed to have.)
For those particular Spitzhaubens, I think two were fine, and the third was probably still strong enough to protect the bird's brain under all normal conditions.
Taking off the skin and poking with a knife works fine on a dead chicken, but of course I wouldn't do it to a live one! For a live chicken, I suppose a person could gently feel with a finger (might detect big holes), or maybe take the bird to a vet for x-ray (probably not worth the money unless you were trying to select birds for breeding and wanted to be sure of nice solid skulls.)
If you are looking for pretty chickens to have in a backyard flock, and want to order chicks with a reasonable chance of strong-enough skulls, I do not think you need to avoid Spitzhaubens (based on my small sample of 3).