Counterfactual variables replicate the entire data set (see ?avg_predictions). So to replicate

mdf <- mdf2 <- mydf
mdf$treat = 0
mdf2$treat = 1
mdf <- rbind(mdf, mdf2)
res1 <- predict(mod, newdata = mdf, type = "response")
mean(res1[mdf$nodegree==0]) # 8290
mean(res1[mdf$nodegree==1]) # 6046