hmm difficult question.
Intercept
If this creature is guarding, and if a ranged attack targets a non-flying object in the same zone, this creature may
redirect that ranged attack to itself, as long as it can be a legal target for that attack.
In my eyes, Combustion is a spell where the victim is the origin of the combustion, there is no projectile the angel can see to jump in between, therefore no interception should be possible.
But, if Combustion as a ranged attack fits in the first part of the interception rule, i would say, that Combustion can only be intercepted, if the Angel is burning too, because a non-burning Angel cannot be combusted.