He did not, someone analyze that mojil condition simply would not be curable by the medicine of that time. Pilgyon simply tried to curry favor and be generous by giving whatever time left of him to soongap. Afterall, by his experience (his father), he could simply take whoever he love and force them to left their actual lover.
Thats why he got pissed off when soongap accuse of him poisoning mojil. His act of "kindness" was not going through the way he planned. He think soongap would just forget about him and filled with gratitude towards him.

im actually curious abt the whole Wolji arc, was Pilgyeon purposely poisoning him? how did he deteriorate so bad, tho tbf it was addressed he was sickly as a child, its just odd