Steering stability means that when a disturbance occur on the steering (riding in a pothole for instance), it self returns to its neutral position (it returns without any need of a rider action on the steering)..

According to this definition, all bikes sold on the market are stable, nobody would sell an unstable bike, i.e. a bike that would crash when a steering disturbance occur (and without rider action).

What makes the difference between very stable and less stable bikes is how fast the steering returns to its neutral position, some do that very quickly, some less quickly. Some steering have some oscillations before staying in the neutral position.

If you look at the Brommie, when the front bag is heavy loaded, it is less stable because it returns not as quickly as usual and if the load is heavy and wide, it may even oscillate once.

Such a behavior makes a bike feel less secure, you want to keep (both) hands on the handlebar while you can ride one or no hands with a very stable bike.

The wheel size, due to the gyroscopic effect, has some influence on this behavior but the frame geometry is the dominating factor for stability. As said, some lightweight race bikes have a very "nervous" and not very stable steering while they have big wheels.

Saying that Helix will be very stable, more stable than a Brompton only because it has bigger wheels is not valid.

The only way to know is to make a test ride with Helix and nobody did it yet with the final/last design that doesn't exist yet and will be very different from the prototype made for the kickstarter campaign.

This is for steering stability which is only one characteristics of the behavior of a bike. There is also cornering ability, road surface sensitivity, wind sensitivity, ability/efficiency to transfer power to the ground... many parameters that make the difference between an excellent bike, a good bike... a poor bike and cannot be evaluated from the bike datasheet.