Umm claiming Eurostar don't try to integrate with other rail services is incorrect especially as National Rail advertises integration between National Rail services and Eurostar services allowing though ticketing from York or Huddersfield to Paris for example.
It may indeed say that on National Rail Enquiries, but unfortunately it is not true, as others have explained.
To be honest we've moved well on from this now. The original point being made was that long-distance rail connections with Eurostar either in London or on the continent can't be booked on one ticket and thus involve risk being borne by the traveller which wouldn't be otherwise, and that this is a disincentive to such use. It's still
possible, yes absolutely. In the same way as it's possible to combine PAYG safely with Eurostar if you're prepared to make your own arrangements should there be an issue, and that it's
possible to get a workaround of a London International ticket in a separate transaction, although these often cost more anyway, and few people will visit a ticket office for one. It's just not likely. And it's also clearly not a business proposition, because otherwise they would be doing something about it wouldn't they?
You do have more luck booking through to 'any Belgian station' though this is easier as Belgium is relatively smaller by travel times, and SNCB don't restrict the trips by time. Oui.sncf can do some partial integration with France too, and Any Dutch Station is apparently coming to the UK version of the site, although it still won't let me today. Presumably this is because of the pandemic delays and temporary timetables.