Usually fiber coatings such as pyrolytic carbon are applied in order to create a weak interphase and thereby to achieve damage tolerant, high strength composites. Can in this sentence 'weak' be interpreted in the meaning of "brittle"?
I don't believe "weak" in this context means "brittle".
The sentence is speaking about composite materials - a material made from two or more other materials with notably dissimilar chemical or physical properties that, when merged, create a material with properties, unlike the individual elements.
As the resulting composite is described in your sentence as "damage tolerant, high strength" it seems unlikely they are adding a "brittle" element to it.
I think the key to understanding this is knowing what is meant by an "interphase". It literally means something between two phases, or stages. That could be a stage of production rather than a 'layer' of the composite itself, but someone within that field would be able to answer confidently. My best guess would be that "weak" in this context means "very slight", or "low power". The end result of this interphase ultimately adds strength and resilience to the end product.