"In particular, it doesn't account for transfers, either direct (social welfare payments) or indirect (subsidised healthcare, housing, childcare etc etc)."

I believe your claim may be incorrect:

"Depending on the country and year, the data refers either to income (after taxes and benefits) or to consumption, per capita."

I think they are trying to place a dollar value on healthcare and childcare provided by the state and incorporate it as "income". I might be misunderstanding. It seems like a very imperfect science, but about as good of an effort as anyone has made.