He might actually be using an even thinner product, sold as a 'cleaning' compound. I have seen clear primer, though ... sometimes at the home centers!
The joint in the picture appears to have used a gray cement, which I have seen sold in the plumbing aisle. The stuff I've seen that was actually listed for electrical use has always been clear. The clear glue is definitely thinner than the plumbing cements.
I will admit to changing my practices according to the conditions of the job site. For example, the listed cement is a poor choice in the summer sun - it dries too fast for you to make up your connections - while it works quite well in cooler temps.
Since we do not usually use a primer, I have also found it necessary to apply the glue to both halves of the connection. I think you really need the solvent to have some time to 'open up' the pipe surfaces before you can expect the cement to 'weld' them together.
Not that electricians pay much attention to PVC joints ... it's not like we're worried about electricity leaking out under pressure