The "old friends" hypothesis is not that parasites evolved to be beneficial to us. They're not literally our friends, that's why "old friends" is in scare quotes. It's that the immune system evolved in an environment where parasites were omnipresent, and it malfunctions in an environment where they are entirely absent.
For example, the IgE protein present in peanut allergy is part of the parasite-fighting machinery of the immune system. It's not supposed to be reacting to food. But when there are no parasites to fight, it doesn't just become dormant as we would like.
However the original article talks about 'peaceful' commensals and links to this post (https://rachel.fast.ai/posts/2024-04-25-microbiome-1/), with the example that "friendly" microbes may not trigger your immune system if they leak into your bloodstream because they 'look similar' to your pancreatic cells.
While critical during the development of the immune system, this idea that there are certain actually 'friendly' or peaceful bacteria is less and less supported by the literature, and overstated to the point of probably being harmful. Even probiotic strains can be harmful if your immune system is overloaded or incapacitated.
Strong immune avoidance or induction of tolerance outside of the critical window is a strategy often used by pathogens to escape host defenses.
For example, the IgE protein present in peanut allergy is part of the parasite-fighting machinery of the immune system. It's not supposed to be reacting to food. But when there are no parasites to fight, it doesn't just become dormant as we would like.