The quite in the article isn't particularly helpful. Thisnis one of the cherry-picked quotes that journalists love to use that sound imagimative and colorful, but explain exactly nothing.

So the field has a self-refulating field containment in the radial direction. This was never a particularly crtitical issue. The issue is building non-leaking mirrors at the end caps where the field needs to separate from the confinement to loop back on itself. You can only shape field gradients there so that they form a potential that particles on escape trajectories have to either overcome or be reflected by. I forgot the exact details, but the issues is that electrons are harder to confine and they gradually build up an electric field that attracts the protons and thus counters the mirror gradient of the magnetic field.