I very much suspect there's a bug here.
The amount they reduced it by (roughly two thirds) is about the same as the potential +Light damage legacy can give when maxed out (+69%).
The problem is that that legacy is not working correctly.
According to the patch notes:
Flash of Light has had its damage reduced. However, this ability will now properly benefit from added light damage bonuses from Legendary Items at higher levels.
Indeed the damage has been decreased. Whereas before the patch I was doing 10,081 to a given enemy, now I am doing 2,880 to that same enemy.
If I equip a belt with +42% Light damage, I do 3,084 -- an increase of around 7%, not 42%.
For that alleged "42%" to come out as instead "7%" means Flash of Light base damage is around 500 at level 130, and that some mysterious trait / stat is giving my Guardian +450% damage to it to bump it up to 2880, and then my +Light legacy is only applying 42% against that 500 base, to up my damage to 3084.
I think that's very unlikely. I think the intention here was that they reduced the base damage by 69% but then assumed the legacy would allow people to recover that 69% … except that's not how the math is working out.