Not spooky. We offer you the freedom to choose the MTA you prefer. Why would we re-invent the wheel and build or bundle an MTA in Checkmk, forcing you to use that one?
The one point that is valid here, is that for HTML mail we offer the above-mentioned “synchronous delivery”, which uses smtplib. That is currently the only place in Checkmk, that uses Python to send a mail. I cannot speak to why that is the case and if it will change at some point.
Checkmk by default sends mails from the address $SITE@$HOSTNAME, unless specified differently within Checkmk. So you can configure your MTA to e.g. rewrite the default mails sender, or just specify the appropriate sender address in Checkmk.