Only napkin math. The typical sources of climate change denial don't see biomass as a threat (Because it is tiny, growing slowly, and augments fossil fuels, not replaces them), so they don't fund any research to attack it.

The proponents of it probably know that its numbers don't look great, so they don't push for research that thoroughly audits it.

It's also difficult to thoroughly audit the carbon costs of a complex supply chain that has to move tens and hundreds of millions tonnes of lumber - when the costs greatly vary based on how the lumber was sourced.

It can be tested in the other direction too: is there research that can confirm or debunk the opposite claim?

to confirm one is to debunk the other of course.