Did you read it?you apparently didn't read this: vet manual, professional version, about poultry worms and wormers, the quote is just a small part of the entry, you might want to read the whole page
https://www.msdvetmanual.com/poultry/helminthiasis/helminthiasis-in-poultry
Yes, that's the first line summary of the article. I can somewhat cherry pick through the article too, let's be clear of that.
Reading further.
I see symptoms listed throughout the article some include:
unthrifty, inactivity, depressed appetite, death, emaciation, granulomas, suffocation, weight loss, thickening of and inflammation of organs, thickening of cecal/intestinal walls, nodule formation, inanition, weak, maceration, gasping, choking, diarrhea...... there's more, but you get the picture.
"""""Field studies show that poultry maintained under free-range conditions may be heavily parasitized; therefore, control measures such as preventing infestations or chemotherapy can improve weight gain and egg production. Prevalence of helminthiasis is greater in poultry raised under nonconfinement conditions than in those housed in cage production systems (1)."""""
I suppose an argument "for" modern commercial poultry operations and confinement - yes?
""""Modern confinement rearing of poultry has substantially decreased the frequency and variety of endoparasite infestations such as helminthiasis, which are common in ranged birds and in backyard flocks. However, severe parasitism still may occur in floor-reared layers, breeders, turkeys, or pen-reared game birds where management problems may exist.""""
What's the solution? Do your best to keep a tidy coop/run, if you free range great. IF at all possible, get a fecal float to confirm the presence of worms. (Not everyone has vet access or can afford a vet). If vet care is not an option, then Treat Judiciously.
While yes, the argument can be made that the birds can bear a worm load and still live, do consider what impact the parasites are having on the overall health of the bird and what, if any damage is being done internally.
You mention that worms don't kill chickens. Perhaps you are partially right on that point. A few worms if the infestation is heavy enough can cause the bird to suffocate (die) or block intestines (die). Others cause internal inflammation, damage to organs, etc., so I suppose technically the worms did not kill the bird, just their presence created an underlying condition/illness/infection (whatever you want to call it) that did kill the bird.
Let's be clear. This is the first line of the entry on helminthiasis in poultry in the MSD vet manual:
"Helminthiasis is infestation by parasitic worms. In rare cases, affected birds develop clinical signs such as apathy or diarrhea. "
In rare cases birds develop clinical signs. Signs such as apathy or diarrhea. Not that worms kill chickens.
https://www.msdvetmanual.com/poultry/helminthiasis/helminthiasis-in-poultry