Compensating for carbon released from fossil fuels by storage in a building or a tree with limited lifespan is always dubious. There should of course be a way to account for and incentivise that temporary storage where it genuinely leads to increased amounts of fixed carbon for significant duration.
Assuming that the wood form the house will end up in a methane releasing landfill a considerable timespan into the future is of course flawed. I would guess that it is more likely to be burned as fuel if it becomes available several decades from now.
"negated the benefits ..by 24%" should probably be read as "reduced benefit by 24%"