You may be right; The Helix has larger diameter tires, but a more rigid frame and no suspension like the Brompton rear. Without the directional layup like carbon main tubes, it can be difficult to get longitudinal frame flex for a good ride, while also torsional stiffness so not like a noodle when climbing standing.

I am assuming the Ultralight has the Hong Kong gearing option? I thought HK option had at least 400%. The problem with Brompton gearing is that low-low or high-high and you're not running direct drive middle range, so you have friction losses inside the hub, which are enough to be noticed. With the larger wheels for better ground clearance, the Helix should have no problem easily exceeding 400%, perhaps even 500% with a standard RD and wide 1X gearing.