It’s GOT to be steel, a way better hardtail ride than aluminium and absolutely noticeable, when you ride the two materials back to back it screams out at you.
Ive owned several steel hardtails -still own 2- a 97 Lava dome- compliant, but a but flexy, a 2008 Unit a lot stiffer but still great fun, playing with sliding dropouts makes a difference at max length it feels comfier (but slower)
My old Muni-Mula felt very stiff compared to the steel bikes i had either side of it
My current main hardtail, a Honzo DL , despite being stiff and hard charging is still incredibly comfy (2.6tyres help!) Im not sure whether its the funky curved seat stays too , I have dropouts about 1/3rd off minimum but its got a great feel
I wouldnt dismiss aluminium out if hand