We're talking about programming languages being memory safe (like fly.io does on it's security page [1]), not about other specific applications.
It may be helpful to think of this as talking about the security of the programming language implementation. We're talking about inputs to that implementation that are considered valid and not using "unsafe" marked bits (though I do note that the Go project itself isn't very clear on if they claim to be memory-safe). Then we want to evaluate whether the programming language implementation fulfills what people think it fulfills; ie: "being a memory safe programming language" by producing programs under some constraints (ie: no unsafe) that are themselves memory-safe.
The example we see in the OP is demonstrating a break in the expectations for the behavior of the programming language implementation if we expected the programming language implementation to produce programs that are memory safe (again under some conditions of not using "unsafe" bits).
[1]: https://fly.io/docs/security/security-at-fly-io/#application...
If you've got concerns about our security page, I think you should first take them to the ISRG Prossimo project.
(In a separate comment about "what do people claim about Go anyhow", I linked the memorysafety.org page, but I did not expect it to help in getting you to the understanding that we can evaluate programming languages as being memory safe or not, where something from the company where someone was a founder seemed more likely to get a person to reconsider the framing of what we're examining)