Is this plagiarism? (Update: jury says no)

Personally, I don't think it crosses the line into plagiarism/copyright-violation.

Yes, the chord progression is very similar as is how the notes fall on the beat, but it's not a particularly complex arrangement. Let's Get It On is in E-flat major and Thinking Out Loud is in D major.

I think this is far more a case of how pop songs aren't (by their very nature) inventive and pop chord progressions over pop beats can sound very similar. But I don't see it as any more than that - and I don't think Marvin Gaye gets to lay claim to that progression and beat for 70 years past his death.