pjmlp's claim is: there are people out there who over-hype WASM, who claim that simply compiling a C program to WASM will automatically make it secure. pjmlp is claiming that these over-hyped claims are wrong (in their words: "programs inside the sandbox can still be tricked into internal memory corruption"). You agree that those over-hyped claims are wrong (in your own words: "compiling C to webasm doesn't fix C bugs within the local memory space of webasm"). I agree that those over-hyped claims are wrong.
If one wants to argue against pjmlp (as you and I both do), there are two ways to go: 1) you can say that nobody is making those claims, or 2) you can say that if people are making those claims, the problem is with those people, not WASM.
I choose #2, because it seems plausible to me that VC-backed startup types over-hype WASM. If I was more familiar with the discourse around WASM, I might try to argue #1 if I thought I could demonstrate that over-hyping WASM doesn't really happen. Maybe you could go the burden of proof route, that it's their responsibility to prove that people are over-hyping WASM and take the discussion from there.
But you ... don't seem to be engaging with the argument. You agree that "compiling C to wasm doesn't fix C bugs within the local memory space of wasm", yet you say that "the claim of insecurity is not being backed up by anything" when the claim of insecurity is that compiling C to wasm doesn't fix C bugs within the local memory space of wasm. That's literally what was meant by "internal memory corruption" in pjmlp's very first comment which mentioned the security angle. Again, they're wrong to mention this as a flaw of WASM because making C code resistant against "internal memory corruption" was never a goal of WASM, but that doesn't mean that their "claim of insecurity" is wrong.
To show an example of the kind of exploit pjmlp is talking about, just take any C program with some kind of exploitable buffer overflow, run it in wasmtime or wasmer or whatever, and exploit the buffer overflow. Maybe the buffer overflow lets an attacker write 101 bytes to a 100 byte buffer and therefore flip an "isAdministrator" flag from 0 to 1. You, I and pjmlp all agree that such security issues can exist; WASM doesn't magically protect C code from memory corruption. And that is not a flaw in WASM, which is what your argument should be focusing on.