Sorry, but you're still well into the "don't know what you don't know" stage of learning, which is _not_ the time to become responsible for a colocated server.
Go home, from the colocation idea.
Start with way less expensive VMs from your choice of hyperscaler or el-cheapo-vms.com, or in a homelab with old hardware or RasPi/IntelNUC type devices.
If you "need" colocation, you need someone who knows what they're doing to set it up and maintain it properly.
Or just go ahead and wing it. Maybe I'm wrong. Maybe Rachel's wrong. Maybe you're "The One".
Incorrect. Wildly incorrect. I used to co-lo servers, it has been a long time. SSH Keys were not in vogue back then. I did use them at one point and, because I haven't done it in a while, it slipped my mind. I'm just rusty.
There's just nothing wrong with adding a few steps to do your keygen, upload your public key, save your private key on a thumb drive or whatever, and so on. Please, tell me why that should NOT be added to a tutorial. Whom does it harm? What is the downside?
I can't see anything but Crappy Gatekeeping, but please, come up with a decent reason not to add just a few more instructions.
(Although now, apparently, keys are out for some folksand certs are coming in vogue. Who knew?)
How many _other_ important things have "slipped your mind"?
This isn't about ssh keys specifically, it's using ssh keys as a canary - an example of a thing someone who has internalised enough Linux sysadmin and security knowledge will automatically get right. It's shorthand for all the other hundreds of things that need doing to properly secure a public internet connected Linux machine.
This is a way bigger thing that adding a few lines explaining ssh keys.
Perhaps firewall config has also "slipped you mind"? Or TLS certs and renewals? Or shutting down unneeded service started by your distro? Or managing logfiles? Or monitoring disk space/memory usage/cpu usage? Or keeping your JVM/Nodejs/Python/PHP/whatever up to date with security patches? Or maintaining your software BOM and all the security update notification channels for everything you're running whether you installed it ir whether it got bundled in as a dependancy of something else you installed? (Think zx or Log4J)
Or maybe's since you're just "rusty", you're looking for all the sysvinit files you remember being important, not realising your chosen distro now uses systemd. Or your previous experience was on machines from before the era of speculative execution attacks, or from when it was considered acceptable to hash passwords using crypt or MD5?
I don't know when you were doing colo servers while ssh keys "were not in vogue". In the late 90s I was flying annual round the world trips to visit our colo facilities in London, New York, and San Francisco - with half a dozen hard drives in my luggage because replacing raid1 arrays was a better/cheaper solution for us than uploading over our available bandwidth over 56k modems or ADSL. All those machines had ssh keys. It was standard practice.
But yeah, maybe you're right and my 30 years experience and hard earned advice is just "gatekeeping". Same with Rachel the article author's decades of experience (at way higher levels than mine). I'm sure that your assertions of it being "Incorrect. Wildly incorrect" is not another one of the things that've slipped your mind or that you're a bit rusty on. Feel free to assume that having ssh keys "slip you mind" doesn't mean you have a lot to learn (or relearn) before being capable of securely/professionally managing a colo-ed server.
Go home, from the colocation idea.
Start with way less expensive VMs from your choice of hyperscaler or el-cheapo-vms.com, or in a homelab with old hardware or RasPi/IntelNUC type devices.
If you "need" colocation, you need someone who knows what they're doing to set it up and maintain it properly.
Or just go ahead and wing it. Maybe I'm wrong. Maybe Rachel's wrong. Maybe you're "The One".