Right, but claims like that are harder to substantiate because they often rely on circumstantial information, like shares bore centers or ancillary components,

Right, but claims like that are harder to substantiate because they often rely on circumstantial information, like shares bore centers or ancillary components, which could be the result of company policy and not indicative of direct lineage. Whereas, tracing the lineage of a Nissan VQ, GM L-series, or VAG EA motor back to the 90s is very easy. Choosing 90s was basically a CYA for a "citation needed" claim.