I see it a bit differently than that.
His equation of timers to a”l is not absolute, otherwise he would asser lights for those who follow the R’ma and M”B for a”l. To be mattir, he is taking the position (in deference to the accepted practice of using timers) that that a Shabbos clock is not amira l’akum l’maaseh.
He would still have assered because of zilusa, except for the fact that some communities in Europe used a”l regularly.
Hence, he holds it’s muttar to use a timer l’chtchilah, without a tzorech gadol, even in the house, but only for lights. However, it is assur to use a nochri, even for lights, except for tzorech gadol.