First, the author is clearly a hustler, as many of these definitely-not-cheap services are being paid for under free credits. Cheers to them for that. If AWS is willing to give someone $5000 to get going, take it and run. I do think that makes it slightly disingenuous to then say one is running their business on the cheap, but if they can get profitable before that well dries up, who really cares and no one is hurt.
Second, and I saw this theme in the comments as well, if these are valuable services, they're worth investing in. SaaS uptime is definitely important, and it would be a silly risk to self-host such a thing. Frugal is a good way of looking at it, but if these tools help ultimately make money, they're worth paying for. I think that ties in to point 1, where the free services are meant to help you prove that out while also entrenching you slightly if it holds. But as someone who is notorious for yak shaving, being able to shove code out the door and not worry about the rest of it is deeply appealing and worth the gamble.