doublespend protection is not guaranteed unless you check for doublespends
if you don't check for them you are relying on someone else to check for them
if *no one* checks for them then doublespend protection is not guaranteed
Discussion
Yes, if everyone checks, no blocks containing double spends will propagate to reach me. My node will check them anyway during the IBD I assume. So not sure what that guy's know how is about.