It is important for you as a network engineer to learn Linux! Why? There are many reasons including:
A lot of network operating systems are based on Linux, or have a Linux shell you can access, or use Linux type commands. The author shows you an example using Cisco, Arista, and Cumulus Linux. Network Automation tools such as Ansible don't run the command node on Windows. You are probably going to use Linux with tools such as Ansible, Netmkio, NAPALM and other network automation tools. SDN controllers such as OpenDaylight, ONOS, RYU and APIC-EM run on Linux. You will find that many SDN tools require Linux. DevOps tools such as git work best with Linux. IoT devices typically run Linux. A new breed of network devices from companies like Facebook, Microsoft and Cumulus Linux use Linux.
There are even more reasons, but make sure you don't get left behind! You as a network engineer start learning Linux. Learn how to configure Linux networking, how to create users and assign permissions, how to install and run Linux services such as DNS and DHCP.
The course uses various GNS3 topologies with devices such as:

Linux Docker containers
Linux GNS3 QEMU virtual machines
Traditional Linux virtual machines
Network devices - you could use Cisco, Arista, Cumulus Linux or others
Do you want to see something else added to the course? Just let me know. I like to get your feedback on ways I can improve the course and add more content that you think is relevant.

Networking is changing. Make sure you keep up to date! All the very best!