The article by Matt Landis you are reading is a step by step guide for one kind of Skype for Business Server 2015 installation, with one Domain Controller and one Skype for Business Front End Server. That's an example of a very little topology. As said in Skype for Business Server Topology Basics:
Before preparing anything else, you’ll want to know you’re planning
for the right topology for your deployment of Skype for Business
Server 2015. The first thing you need to decide is if you’re going to
have an on-premises deployment of Skype for Business Server 2015, or
if you’re going to combine this with a Skype for Business Server
Online deployment in a Hybrid deployment.
There's also information on which server roles needs to be on a separate server (server collocation).
Then, if you look at Reference topologies, you'll soon notice that even the reference topology for a small organization has several servers, and the topology map only contains Skype for Business server roles; the AD server roles falls outside the map. The small organization also has 4000 users.
These are all indications that if you have a tiny organization with only one server:
- The Skype for Business Online, i.e. same as a service, is designed for your organization.
- You should still have at least a backup domain controller.
If you still wish to use just two servers and do as Matt Landis guides you, Microsoft Hyper-V is free; you can separate the roles on different virtual machines on a single physical server.