Hi,
IMO, with EX in VC and MC-LAG configuration this would be supported as well since the priniciple mentioned before is being obeyed.
With weight of 255 for each individual interface and losing one of the EX, both SRX nodes would be in ineligible state [threshold 0]. This does not seem to be a desired and recommended configuration in this scenario.
Had the weight been equally distributed, 1 of the physical EX going down would not trigger an SRX reth0 failover as the threshold would not reach 0.
Cheers,
Ashvin