Linear forms from the Gowers uniformity norm

This is a companion note to our paper 'A relative Szemer\'edi theorem', elaborating on a concluding remark. In that paper, we showed how to prove a relative Szemer\'edi theorem for $(r+1)$-term arithmetic progressions assuming a linear forms condition. Here we show how to replace this condition with an assumption about the Gowers uniformity norm $U^r$.