@Aaron Thank you for reaching out to us, researched on your ask - WHEN devices were onboarded to defender, like a date and time, not first seen.
As per this document - https://video2.skills-academy.com/en-us/microsoft-365/security/defender-endpoint/machines-view-overview?view=o365-worldwide which you already referred as well and in the mde portal I see only first seen schema has data & time related to the device.
Have you integrated defender for cloud with defender for endpoint? - Defender for Endpoint sensor is automatically enabled on Windows Servers that are onboarded to Microsoft Defender for Cloud.
As far I am aware this info is not possible to get as we don't have such schema defined in MDE for this ask/we have only first seen defined in schema to give details when the device was reported to MDE.
Would recommend to post the same ask over here -https://techcommunity.microsoft.com/t5/microsoft-defender-xdr/bd-p/MicrosoftThreatProtection and also you can share the feedback - From any part of the portal, select Give feedback regarding the same.