Assuming means taking something to be true. It does not mean that something necessarily is true. There is a dictionary entry that attests to that, I believe.
The first dictionary entry I found says “something taken for granted; a supposition.” I guess the age of the dinosaurs is an assumption the same way the earth going around the sun and disease being caused by germs are assumptions. I disagreed with the original statement because, as used by anti-evolutionists, the implication is usually that it’s not based on evidence, more like a premise or postulate.