I think the limitations are written to keep it simple.
Remember Flex thrust could be only slightly below Max thrust.
The engines are not going to fall apart if you exceed these times but why would you want to exceed 5 mins on 2 (all) engines before reducing to MCT or less? The 10 min limit for engine out situations is to allow for your much reduced rate of climb.
I'm on A300 with GE engines, but the same limits apply.
Hope this helps.