They are correct. You’re not wrong, whales are indeed mammals. But “fish” isn’t a monophyletic grouping, meaning you either need to give up on using it as a category or you need to correct it by including everyone sharing the same LCA, meaning mammals would be part of your “fish” grouping.
Of course, that’s how it works within biological classification. Colloquially, you can call them anything.
They are correct. You’re not wrong, whales are indeed mammals. But “fish” isn’t a monophyletic grouping, meaning you either need to give up on using it as a category or you need to correct it by including everyone sharing the same LCA, meaning mammals would be part of your “fish” grouping.
Of course, that’s how it works within biological classification. Colloquially, you can call them anything.