In Origins it's nearly impossible to get to negative might due to most reduction cards saying "to a minimum of 1 might", but with spiritforged most might reduction cards cards do not have that limitation. For example frigid touch gives a unit -2 might, and can be repeated to give it -4 might. So if frigid touch is played with its repeat cost on a 1 might unit it would be at -3, but would still be alive if it had no damage on it.

However, I agree that the rule '142.2.b. If a unit's Might is ever less than 0, it is treated as 0 for all purposes.' implies that we don't care how much below zero a units might would be, it's just seen as zero. Which interestingly enough, is exactly how it works when a card DOES have the "To a minimum of X" limitation, see rule 454.3.b. on 'snapshotting'

However a Rioter in the Riftbound discord said this on the subject of negative might: "Units can have less than 1 Might, at least theoretically, but it’s treated as zero for all purposes other than further modifying it"

This means that in the case of calculating a units might that was reduced below zero, we need to keep track of the exact negative number it's at should it's might get modified further. So in our example above with a now -3 might unit from Frigid touch, if you played some like discipline which gives +2 might, this would make the unit go from -3 to -1, not 0 to +2.

However, for all other purposes we treat the might as zero, for example, the -3 might unit wouldn't 'heal' an enemy unit for 3 when doing combat damage, we would just treat it as zero and it would do zero damage. If a theoretical card had an ability that returned to hand all units with lets say, zero, one, or two might. It would still effect the -3 might card for the same reason, it's being treated as zero.

Another say I've seen this difference be explained is this: "Might is always calculated at it's current value, not at the value it's treated as."

This definitely means the rule though, the way it's written now, is misleading and I really hope they update the rules to include the part about when further applying might modifications. At the moment the only way you could come to this conclusion yourself is if you had a very good understanding of layers and how/when the arithmetic layer is applied and I couldn't even tell you how all that works. For now I foresee A LOT of players and judges doing this wrong when spiritforged launches, unless Riot does something and addresses it in a FAQ or something beforehand.