Just to be clear, no one has ever made a bomb with HALEU, so the threat is entirely hypothetical?
They seem to be saying that the bomb would have to be made with a ton or more of material, which suggests that it might be much trickier to achieve a chain-reaction state.
That is, the original bomb only had to move 140lbs of material to achieve a reaction, but a bomb made of HALEU would have to shift more than a ton ... very precisely. all at once.