My wife calls my fifty year old poncho liner my "Security Blanket".
Back in 1957, I went to Philmont Scout Ranch in the New Mexico part of the Sangre de Christos range. We had Army surplus shelter halves while we were hiking and Army surplus wall tents when we in base camps. My sleeping bag was a nylon surplus mummy bag outer liner with a wool inner liner. It rained almost every day we were hiking and all but one of the four days we stayed in a base camp. I carried my sleeping bag on top of my pack rack. After 2-3 days on the trail, the other guys' department store sleeping bags were all wet. I had been rubbing it in that my 10+yr-old $5 surplus bag was staying dry. We were hiking next to a creek and the guy behind me cut the ties holding the sleeping bag and tossed it in the creek. I bobbed along until the trail got close the creek bank. I leaned over and picked the bag up, tied it back where it rode. That night when I unrolled it, it was still dry. Sounds a lot like your poncho liner.