It did look like he poured a lot onto the glass, but afterwards it didn't appear that much was really used in the bottle. I have not used the glass sealant before, but I would imagine as long as you can see a thin layer being applied it will be just as effective.
As far as paint sealant goes, you want to start with a clean clay barred surface prior to applying sealant. Once it cures and you top it with wax you will still be able to use detail spray without ruining the protection you put on the paint.