[tombert]

> Do not confuse lack of proof as proof of lack

If taken to the extreme, this logic can be used to prove literally anything. "Haven't seen any proof of invisible back-massage fairies? Well, don't confuse lack of proof as proof of lack!". When your framework can be used to prove something false, it means that the framework is unreliable.

I don't really know what "sciencism" is, but we know that the scientific method has been demonstrated to work in producing results that are useful to us. I am typing this message on an electronic keyboard, plugged into a computer, plugged into a giant-ass interconnected network hitting satellites and fiber-cables and whatnot, all of which were developed by scientists or engineers following a scientific method.

To use your wording, however, while I can't conclusively say "lack of proof is proof of lack", I can easily say "lack of proof is strong evidence of lack". Occam's razor tells us to discard unnecessary assumptions.