At the risk of being pedantic, this is conceptually, but not technically, correct. I’d describe it as Proxmox is an operating system like Windows is an operating system. Proxmox is highly efficient with minimal overhead (unlike Windows plus virtualization), free (for personal use) and open source. Proxmox provides virtualization capabilities so you can easily run HA and other things without any Windows layer underneath. (Among other benefits, this means no Windows license cost, and no unexpected reboots due to Windows updates.)
I knew nothing about Proxmox until I followed the excellent Community Guide here, but I found it simple, powerful, and stable, and have been very pleased with my HA environment since installing it using this method.