The fact of the matter is the invariant sections ARE a restriction making it completely not a total freedom content license, but if you look at that from another angle, it gives total freedom to everyone who publishes a document, the free content mostly corresponds with the fact the license stops reference books from costing anything monetary wise.
Yes, the GFDL is a free content license when used with invariant sections, because the person wishing to use the content can still freely use the content. The person can use the content freely, they just cannot make changes to it. That does not mean that it is not a free license. The restriction is on modifications, not on use in general, so it still meets the definition of a free content license.