IS, the Shevet Haleivi is going according to the Mishnah L’melech and Noda Bi’Yehuda that swallowing without chewing is still considered derech achilah. The fish oil is still not derech achilah, because it’s not a substance consumed on its own.
If fish oil became a substance which is consumed on its own, then although according to the Toras Chaim’s definition of shelo k’derech achilasan it would still be muttar to swallow it as a pill, but would become assur according to the ML”M and NB”Y.
I am not sure if he is only m’supak if it changed l’issur because he’s not sure that it’s common enough to be considered k’derech achilasan, because he’s not sure that even according to the ML”M the proliferation of swallowing pills makes something as a pill makes it kd”a, or because he’s not sure how to pasken in that machlokes.