the hdr box enables some roll off features, but disables luts. imho, its kinda worthless right now because you can’t actually grade in rec. 2020. the lumetri color engine doesn’t support any color management besides sRGB/rec. 709, and you can’t export codecs in ST. 2084 gamma in true rec. 2020 color.
sRGB and Rec. 709 are the same thing if the rec. 709 is 2.2 gamma. they have the same color primaries. there are differences in 16-235 luma and 204 sat color for broadcast limits.
if you use other tools that are not lumetri, it may be possible to grade in higher than sRGB in AE only(not premiere). opencolorIO plugin is one example because it supports larger gamuts in ACES colorspace.