Clearly someone doesn't know what a presumptive test is.
It's a test which is quick and easy and responds positively to what you're looking for plus possibly quite a few other things. e.g. there are several presumptive tests for blood which are actually tests for peroxidase* activity, blood being one of several substances, including fruit juices, which have this property. So although a stain might give a positive result one would have to follow it up with a more specific test for blood. In my day it would have been an immunoelectrophoretic test which would have confirmed whether it was blood but also blood of the species (usually human) that you were testing for.
The key thing is that all it does is tell you to look further. It is not of evidential value.
*We discarded one of these as a component of the kit was known to be carcinogenic. I wasn't happy when years later I discovered from my daughter that it was still being used with enzyme-linked antibodies as a microscopy stain.