You understand it incorrectly

“ In July 2013, Internet Service Providers (ISPs) agreed to voluntarily offer “default-on” adult content internet filters on all new and existing home network customers” [1]

OK, so the entire industry does opt everyone in to content filtering by default, just every single provider, without exception, does it “voluntarily”.

1. https://researchbriefings.files.parliament.uk/documents/SN07...