The claim by R&R is that yearly GDP growth decreases abruptly (i.e. in an non-linear fashion) when debt exceeds 90% of its GDP. According to them, therefore, governments should have, in times of recession, debt reduction at the top of their economical agendas.

The response by Herndon doesn't insinuate that countries should disregard debt as a matter of concern. It doesn't even say that debt increase isn't correlated with GDP decrease. The main issue that the paper points at is that US and European governments have, based on this thesis, adopted drastic policies of austerity and budget cuts, when actually over history there isn't such a nonlinearity.