DHH is a salesman/marketer above all else so you need to take anything he says with a grain of salt.
That said he is right on several crucial advantages of running your own kit:
1. It's a lot cheaper, once you reach the scale of 2-3 racks (or roughly 30-45kw of gear or so) you are well in the green even if you need to hire additional staff to manage the HW side of things.
2. Performance is on a completely different level especially if your workload benefits from I/O. Being able to stack at 2U box with an absolutely silly amount of NVMe drives makes light work of what would otherwise be tricky scaling challenges.
3. Access to real raw networking makes a whole bunch of things more optimal, namely real BGP (yay anycast) but also better load balancing primitives like Maglev.
Now these are all very good reasons to want to be on your own gear but DHH completely glosses over the very real disadvantages.
1. As time goes on you need new hardware, either to replace gear that is out of service life, has otherwise failed or most commonly for expansion. This will require doing the same validation you did when you first ported off (or built out if you were greenfield) because it's very likely you aren't going to be able to get the same SKUs with the same BOM. So over time your infrastructure becomes less and less homogenous which increases the cost of management and becomes a burden in terms of operational knowledge. TLDR introducing new hardware never gets easier, the more unique SKUs you add the less fun things become.
2. Networking is generally entirely glossed over in posts like these. Running your own network with your own AS, IP space, edge routing, dealing with peering and transit, balancing upstreams etc is work. You might be able to offload this to your DC provider in some cases but usually you are going to have real network engineers on staff, you can't repurpose a curious programmer into a network engineer (atleast quickly). That is also ignoring the fact that most network gear still hasn't caught up to the age of IaC. If you want that you will be spending either more money (Arista/friends) or paving road ahead of yourself (unless things have changed in the last few years I have been away from gear).
3. The financial aspect is as a whole a lot cheaper but you trade total net $ for complexity of financial arrangements. Often times you will want to lease servers, you need DC leases, you need transit agreements etc. You went from a single entity you pay in "The Cloud" to many different vendors. Vendors suck of course, lots of vendors sucks even more. If you are a larger company with a procurement office this is a non-issue or can even be an advantage because said procurement team can put the screws into vendors individually.. so YMMV depending on size of your company.
4. While most older software engineers and infra folk generally have bare HW experience the newer generation (last 10 years) for the most part have never touched anything lower level than a VPS. The days where every infra guy knew IOS, megacli, etc like the back of their hands and wrote Perl one-liners instinctively has sadly passed. You will need to seek out actually qualified engineers. This can put off businessy/less-technical types because they don't like the idea of not being able to hire a bunch of juniors that know close to nothing and will accept peanuts for pay. If you want to to do this but you are coming from the technical side (you want them sweet IOPs) this is probably one of the bigger organisational barriers you will probably face.
4. Things fail. Often at very inopportune times. Runbooks, backups, DR, etc these are all much higher stakes on your own gear. Culturally if you want to go this route you also need to be willing to accept a higher level of investment in these and associated activities like running proper drills. You should do this anyway but in the case of "The Cloud" you are getting higher durability etc that do reduce risk vs your own spinning rust/SSDs and operator error.
All that being said I'm still on team "run your own gear after $200k/mo bills start showing up" but I don't like DHH painting such a one-sided picture of what this really entails.