Coal can't be shut down easily, it's incredibly expensive and inefficient to dial it up and down. Natgas is easier but still quite a slow process.
If you could turn gas and coal up and down in a few minutes, then perhaps you would have a point. But coal can take 6-24hrs and natgas 1-4hrs. It won't suddenly stop when you have a massive gust of wind.
Wind and solar generation are highly predictable on those timescales, you can find predicted and actual generation graphs from various sources. They're certainly more predictable than demand.
More realistic excuses I've heard are long term contracts, potentially with minimum run times for fossil plants. Which are therefore the actual problem at the moment.
If you could turn gas and coal up and down in a few minutes, then perhaps you would have a point. But coal can take 6-24hrs and natgas 1-4hrs. It won't suddenly stop when you have a massive gust of wind.