the poverty line as the 40th percentile
of income [...] was absurd.
In the past (19th century and before), the socialist/communist tradition had a real point in that large swathes of the population were poor in an absolute sense (e.g. going hungry, cold, no adequate health-care, pension etc). The socialist/communist tradition was in some sense about abolishing absolute poverty (whether the proposed methods were helpful or not is another discussion). If you read e.g. the communist manifesto [1], that is clear. Most of socialist/communist politics centered around politicising and radicalising the absolute poor with a view towards taking them out of absolute poverty.
However, modern industrialised society has (simplifying a bit) abolished absolute poverty.
This leaves socialist/communist politics in a conundrum: how to continue to politicise/radicalise people to become followers. The concept of relative poverty is the answer: all the old political tools can be reused by changing what is meant by poverty.
[1] https://www.marxists.org/archive/marx/works/1848/communist-m...
And I thought the US move to define the poverty line as the 40th percentile of income (or something lower; I don't remember this too well) was absurd. The average household income is, by necessity, well above the 50th percentile everywhere. There is no one seriously advocating that over 50% of every country's population is living in poverty.