While “having fur and producing milk” may have been the original description of mammals, biology no longer classifies things that way.
Today, all of the classification is based on Clades, which can be established by doing DNA investigation (similarity matching). So, “mammals” are defined as a clade. And whether somebody falls into it or not depends on their DNA similarity. And coconut clearly do not belong into there.
While “having fur and producing milk” may have been the original description of mammals, biology no longer classifies things that way.
Today, all of the classification is based on Clades, which can be established by doing DNA investigation (similarity matching). So, “mammals” are defined as a clade. And whether somebody falls into it or not depends on their DNA similarity. And coconut clearly do not belong into there.