What worked for me was I installed and ran Thunderbird, then I quit and copied the Google Takeout .mbox file to ~/.thunderbird/xxxxxx.default-esr/Mail/Local Folders. When I re-launched Thunderbird, it took a few minutes and then all my Google Takeout emails were browsable/searchable under Local Folders.

Also, I think there was a weird step involved, like I needed Thunderbird to connect to some email account, any email account, in order to get past the welcome screen.

For anyone curious, It’s Always Sunny in Philadelphia S04E11 “The Gang Cracks the Liberty Bell”