This is quite far off the mark.

Many companies using FreeBSD, including Juniper, NetApp, Netflix, Netgate (pfSense), iXsystems (TrueNAS), Dell (Isilon) contribute significant code to FreeBSD. It's very expensive to maintain long-lived changes from upstream, so there's a large incentive not to do so. Code that's "not contributed back" is largely code that isn't suitable for upstream anyhow - because it is incomplete, limited in scope, etc.

Looking at "Sponsored by" tags on the last 6 months of commits to FreeBSD I see the following:

  The FreeBSD Foundation
  Netflix
  Rubicon Communications, LLC ("Netgate")
  Chelsio Communications
  NetApp, Inc.
  Mellanox Technologies // NVIDIA Networking
  Innovate UK
  Klara, Inc.
  Diablotin Systems
  Dell EMC Isilon
  iXsystems, Inc.
  Citrix Systems R&D
  Axcient
  Netflix, Inc.
  DARPA
  Alstom Group
  Eldorado Research Institute (eldorado.org.br)
  Ampere Computing
  Marvell
  Stormshield
  Amazon, Inc.

(and a long list of entries with one or two commits each)

There's a backlog of work that contributors would like to get into FreeBSD; a limiting factor is availability of mentor and reviewer time to guide contributors through the process and iterating on bringing the code into a committable state.