Most mailservers don't send emails one character at a time. You appear to have a mental model of how a spam filter works that doesn't include fingerprinting the behavior of the sending server? Even open source mail servers do that, for example postscreen is a part of postfix.

I wonder if it'd fare better if I scripted nc with expect?

(Not sure I care enough to try...)

Where it came from is one of the most important details!

Is this mail relay actually elided.org or is it impersonating elided.org?

There does not appear to be an SPF record for that domain so Gmail cannot determine if this is a fraudulent message, especially if the machine this request came from does not match the IP of the A record for that domain.

Some of the things it doesn't contain, though -- like Message-ID and Content-Type headers -- are likely to contribute to its spam score.

If your domain has SPF headers which don't permit mail from the (residential?) IP you're connecting from, that'll be a major factor as well.