We have no clue what the upper limit of Shanks's and Roger's CoC is. Saying they can't have JB level Haki because they never did something like reverting multiple Gorosei to human form is fallacious because they've never been given the opportunity to do something like that in the story.
Will not come to conclusions unless we've seen everything both sides
But it is the portrayal that sets everything apart without needing much to dive in.
Roger is a legend and his portayal is the highest one tied with WB( and also Rocks)
There's a mysterious figure called Joyboy.
And the portayal of this person is a whole new lvl. Roger and others look like side character infront of this person
Ofc he has every rights to have unparalleled strength both haki and whatever.
But will follow first paragraph.
It's a big question if even current Joyboy would surpassed his haki but more believable be on same lvl. Coz why?? Both are identical person that shares same dream, same ambition, same power etc.
And let alone people of the lesser D on equal lvl with the two ........especially the person not even have any relation to Joyboy or D family.