That's a theory, anyway. Yes, biologists (and presumably botanists and all those who study the other kingdoms) have made errors in judgment, and yes, the current definition is very poor (often broken, sometimes unprovable in a practical sense).

It does not follow, however, that all life exists on some continuous spectrum of mutations. Members of Equus caballus have distinct genetic traits that do not occur in Equus asinus.

The reality we see about us is that life seems to group itself into channels or pools of limited genetic variation. But that variation doesn't mean the channels/pools aren't real.