Not true. They only have to behave "reasonably" in a delay. Unless you can somehow prove that their actions were not reasonable, you are not likely going to win any compensation. Here's a good summary --
" However, Article 19 further provides that, the "carrier shall not be liable for damage occasioned by delay if it proves that it and its servants and agents took all measures that could reasonably be required to avoid the damage or that it was impossible for it or them to take such measures." "All measures" has not been interpreted to mean an airline must do everything in its power to avoid delay, only what is reasonable."
"Courts have found that airlines behaved reasonably in delay situations caused by increased security measures, mechanical failures, and weather disruptions. "
I would encourage you to research case law on the topic. Liability would not flow from the Convention for things such as airport shutdowns due to security, the mass grounding of the MAX, or inclement weather, so long as the carrier did all that it could reasonably do to get the passenger moving. If an airport is shutdown, for example, there isn't a whole lot an airline could do. However, as both US and international case law shows, the Convention protects all damages
reasonably flowing from a delay on an eligible itinerary for things such as mechanical and staffing delay, as reasonable precautions have been intrepreted to include good maintenance standards and staff scheduling.
You are correct that a few cases have found that "unusual" mechanical fault is an exception to the Convention, but those cases are sparse and typically apply to cases like "device X was rated at 1000 hours and failed at 100 hours, which we could not prevent with good maintenance." A typical mechanical delay, such as the usual failing of a mechanical device or performance of maintenance, is a covered loss under Article 19.