No, its not.

> The poster is making a positive claim without evidence.

True.

That doesn't make the alternate positive claim you have posited into “the null hypothesis”.

A null hypothesis is null. What you are stating may be your prior, but it is not the, or even a valid, null hypothesis.

> This is a rational use of ‘null hypothesis’, but it also matches the scientific use,

“Null hypothesis” is a very specific scientific term of artz it has no other meaning.

And, no, the specific counternarrative presented here does not match the scientific use of “null hypothesis”.