You should give him the toothpaste, it's not allowed per their rules. You are of course 100% correct that you have less than 100 mls of toothpaste in the tube, but they don't do that math to convert from weight to volume, if the volume is not listed they go weight.

From their blog post about it

Some people have asked why we don’t convert the net weight of the toothpaste to volume since they are different. Good question. The 3.4 container/volume rule was created to make it simple and streamlined for both passengers and our officers. As you could imagine, taking weight into consideration would be a wrench in the spokes. I’m sure the public doesn’t want our officers using scales or conversion charts, etc.

In other words, don't ask them to do any math.