I think your brother-in-law must like the brighter look of the picture but is missing the point of HDR. If Mando was graded in SDR then it should be flagged as SDR on D+ so an Apple TV will correctly play it back in SDR and let the TV/Display use it’s calibrated SDR setting.
Playing back SDR in an HDR container results in an erroneous playback experience due to unpredictable gamma curves and dynamic tone mapping in the absence of DV metadata. I’m guessing someone at Disney incorrectly made this decision because they graded in DCI-P3 and wanted to master in Rec. 2020 instead of collapsing into Rec. 709.
This is an awkward time as the industry learns how to properly transition from SDR to HDR, but if creators want Rec. 2020 then they really need to also go all in on HDR as well to ensure a consistent playback experience. There’s no consumer standard for using Rec. 2020 after all without HDR.
Mando is SDR (in an HDR container) but its brightest elements are 200-nits (should be 150-nits). This was just a bad decision no matter which way you look at it and is what results in the washed out appearance (elevated near blacks)
With all that said, I think this could start a decent argument for using DV for 150-nit SDR masterings that takes advantage of the expanded Rec. 2020 color space because of the tone-mapping assistance provided by DV. Mando must look especially bad on non-DV HDR TVs running automated tone-mapping algos that expect 1500-nits... cough... Samsung... cough...