I'm actually confused here. Stonogo is using enough detail that they sound like someone who understands the domain, but what they're saying is, in many cases, simply factually incorrect.
A few corrections:
1. MTA-STS does not specify ports. I don't even know where that comes from. Mail continues to be sent on port 25.
2. The claim that "nothing is gained over security the MX record to start with" is, again, simply untrue. DNSSEC alone does not solve anything, unless you also somehow validate the presented identity of the recipient MX--which is what DANE and MTA-STS attempt to provide. DANE arguably does this better (neglecting for now the debates about roots of trust and DNSSEC), but, to be perfectly clear, DNSSEC alone (or anything else that merely secures the MX record) does nothing to validate the recipient MX's identity (meaning you're vulnerable to e.g. BGP routing interception or other active MITM).
3. MITM attackers cannot merely RST connections to STS-specified ports. Among other reasons, because STS doesn't specify ports (meaning this criticism is nonsensical) but, assuming "ports" was supposed to read "hosts", because STS requires senders to validate the host's presented certificate. That's sort of the point of that.
I think the criticism here was written by someone who knows a lot of the context but seems not to understand the threat model. If you don't understand the threat model, yes, STS is going to look crazy.
I don't love MTA-STS. It's an imperfect compromise, which is sort of the nature of compromises. Stonogo does fairly contrast it with two alternative approaches: DANE (not just DNSSEC) and simply announcing a planned deprecation of unauthenticated SMTP (which, as an aside, would also require some currently-nonexistent principles on MX presented identities, like that the identity must match the @hostname of the destination mailbox--a constraint currently commonly violated).
But it's trying to solve an actual problem. And, as I said, the heart of the criticism here is simply factually incorrect, whatever warts STS does indeed have.
