Apple forgot to add a Siri command that would launch Voice Memo.app and start recording. You can launch it with Siri, but it won’t start recording.
They did add a Siri Shortcut though - “Record a new voice memo”. So I finally found a use for that feature, I added “Record voice memo” and “Voice memo record” as Siri shortcuts. I think the 2nd one works better.
“Record a memo” doesn’t work because Siri uses that to open Voice Memo.app without launching the recording (iOS doesn’t warn about this during shortcut creation). The behavior is also a bit buggy, sometimes the phrase launched Voice Memo.app but didn’t start recording.
I tried creating a springboard Shortcut, but even though “Record a new voice memo” is in Siri & Search settings shortcuts, it’s not available in Shortcuts.app. (btw, deleting a Shortcuts.app shortcut uses an insanely weird UI).