To quote Software Freedom Conservancy:
For approximately twenty years, Red Hat (now a fully owned subsidiary of IBM) has experimented with building a business model for operating system deployment and distribution that looks, feels, and acts like a proprietary one, but nonetheless complies with the GPL and other standard copyleft terms.
Obviously they comply with the GPL, otherwise they would get sued. But Red Hat acts exactly like a proprietary software company. That’s what the quote is trying to say.