Well, not necessarily true. Your body is not composed of 100% fat. Actually it is composed mostly of water and when you gain 4g of mass most of this gain is from water, too.

I unfortunately can't tell you how efficient body is at digesting various types of foods including digesting pure fat.

Very short-term fluctuations in weight are usually caused by retention and flushing of water. Gaining 30kg over the course of 20 years is not water weight. If you were retaining that much water, you'd be so arthritic feeling I'm not sure you'd able to stand up. Unless you've been very serious and consistent about weight training that entire time, that 30kg is virtually all fat.