FreeRDP is not quite reverse engineered, the software built based on open specification for RDP[1]. That being said, I would be surprised if the FreeRDP contributors did not need Wireshark during development.
I guess they just decided the freerdp codebase (which already runs on Linux) was easier to work with than their codebase (which is deeply integrated into all things windows).
I think the original Microsoft codebase also uses stuff licensed from a third party, so it might have been impossible to opensource.
Well, migrating to WSL2 brought back memories when I moved from conventional HDD to SSD. Also, that might be how people feel when switching from Intel Mac to M1 one.
Possibly? Initial commit on FreeRDP is from 2011 while the oldest version listed on Microsoft's website is v0.01 from 2007. Not sure if that version of the documentation was publicly available.