I think it refers to extra integration features like window resizing, clipboard read/write, drag-n-drop files between guest and host, etc.

Those features are from VirtualBox Guest Additions.

VirtualBox Extension Pack, on the other hand, provides support for USB 2.0 and USB 3.0 devices, VirtualBox RDP, disk encryption, NVMe and PXE boot for Intel cards.

Then he is wrong. That's GPL, you can get it installed straight from your distros's repo.