For many the fight against the Haskallah was not about Reform/Conservative approach to Halacha but rather against Western ideology. Specifically Western Philosophy applied to Judaism.
So a Frum Maskil would be someone who keeps (mostly) Torah and Mitzvot yet regardless studies Western Philosophy is involved in politics and mixes those with the Torah.
See מעין גנים by the מהרצ"א (Ma'ain Ganim by Rabbi Zvi Elimelech Shapira of Dinov (1783-1841))