It was definitely not one of my better worded comments :)
colo == renting space/cooling/power from a datacenter provider to run your own servers.
on-prem == on premise installs/support
They have two revenue models, SaaS and Selling you support [1]. Giltlab has a "loss leader strategy" like many online businesses. The difference is that in this case, their free plan will grow so exponentially fast, it will become a huge cost center.
Also, to add to my simplistic post above, colo wasn't meant to only imply Gitlab having to run everything, for example, they could go with a managed provider, and have them run the network/hardware site of things. This is also still substantially less expensive than the cloud, when your biggest bills will be storage and bandwidth ( which of course, is my assumption ). From their tech postings, it seems their problem domain doesn't allow them to utilize s3 or any other other massive scale distributed storage, so they have been primarily utilizing block storage.[2][3]
He's talking about where GitLab should run their services from. He meant GitLab should purchase hardware and move into a colo rather than run off of cloud services.
I believe he means that the "SaaS option" is run in the cloud by GitLab itself, which is probably very expensive given that the nature of the service is both storage and network-intensive. If GitLab operated their own datacenter(s) it could be considerably less expensive.
Colocated hosting refers to a kind of hosting where Gitlab puts their own hardware inside of a data center. They primarily pay the data center for physical space (also called rackspace), bandwidth and power but they don't pay for the the server itself since the physical hardware belongs to you.
Dedicated hosting is similar to colocated hosting with the only difference being the physical server is rented to you by the data center. This is different from cloud hosting because unlike cloud hosting, with dedicated servers you are the only customer using the physical server that is located in the data center.
An off-shoot of dedicated hosting is a VPS (virtual private server.) VPS can be seen as a mix of cloud and dedicated in that the physical server's resources are split between x customers but each customer has their own virtual machine with root access.
We run their on-prem in our colo, and it works great for us.