OK here is the response I got from support on this. It doesnt explain it from the technical side, but in terms of practicality it does:
"Hi, here's the official word:
For Helix Floor, Rack, LT, and Native, it’s two new cabs per path (so 4 total). However, once you add two, you can still access the older Hybrid cabs.
For HX Stomp and Stomp XL, it’s two new cabs total, but you can still add older Hybrid cabs. (A new dual cab counts as two)
Basically, instead of the logic paying attention to the total number of cabs, it pays attention to the total number of cabs of each type. Amps follow the same logic—two per path."
Since my brain had been initially conditioned by the Stomp, I misunderstood how paths worked in Helix Native, now I can easily do what I wanted, which was to compare new duals to IRs in the same patch.