He hung his argument on the fact that the Shulchan Aruch says that we are m’vatel talmud torah for megillah laining. “But how,” he asked, “could you say that? Megillah itself is Torah, no?” He then answered that since we could be learning G’mara or other more complicated matters, the megillah reading was actually bittul Torah (albeit required).
No, I didn’t buy the argument then, nor do I now.
It’s the Bais Ephraim’s second teirutz to the kashya asked to him by the Noda B’Yehuda’s son.