I have no experience in game development, but have 25 years of experience in leading software projects, from small to large. I've seen all types of software developers and all kinds of views that they have on their skills passing by. Simply put, developers suck at both qualitative and quantative testing on a product level. They need others to take the lead in that. Obviously it is a cooperative thing with the developer, but a developer will never find the amount of issues in their product that somebody skilled in testing can, so they should definitely not be in the lead. Testing is a skill that not everybody has and most developers don't (beyond unit testing their own work, which is something a developer should do), as it is requires a very different skill set from developing software.
The amount of developers that think they are good in testing on a system or product level, but in reality are not, is staggering. With the results of the last weeks, the broken bug fixes, save games, and whatnot, yes I conclude that Cleve is not good at testing his product.
Fortunately for him, he has paying customers who are fine with it being released like that.
There's no such thing as a "tester" in terms of a human being. That's a job title.
Meaning, a tester can be a great developer and a developer can be a great tester.
There's absolutely no conflict there. That's in your mind.
Also, when it comes to overall testing - it's 100% common practice that the developers (many of them) take part in the process. In fact, they do this DURING development every time they're experimenting with something - and they do it as part of the job, it's just not the primary part of their job.
Beyond that, it's very common to have after-work playsessions that are primarily about testing balance and "fun" - and that WILL include developers.
I know this, because I know game development.
When it comes to traditional Q/A testing - as in, quality assurance, that's largely about bug fixes and making sure the game actually works. Severe balance issues are a natural part of it - but it's really the job of the designers (and especially the project lead) to balance the game, and then give directives to Q/A in terms of what to look out for.
All that is kind irrelevant here, though - as we're not talking about a normal game.
We're talking about a one-person game - and there's really no alternative to Cleve testing the game that's realistic, considering the scope and time in development.
Is he necessarily a great tester? Of course not.
Is there some kind of rule or standard that makes him a bad tester exclusively because he's a developer? Of course not.
Whatever your opinion of testers may be - it seems both ill informed and biased.
The reason developers don't usually focus on testing games is that they're busy developing. The reason you use testers is NOT that they're not developers - it's because you need them to focus on that job, and not another job.
You and I could be fantastic testers - or we could be fantastic developers. We could also suck majorly at both.
You can't know from looking at a job title.