Per the guidelines, digital preview dates are typically ignored; since this was broadcast on Channel 4, TMDb uses the broadcast airdates.

From that page:

When a television network airs a preview of their television series on their own network, the preview date can be used. For example, the pilot of Freeform's The Bold Type previewed on Freeform three weeks before the official series premiere.

This seems like the same situation? Channel 4 released the show on their own digital service. Note also that they only released it digital first because they had to delay the original broadcast date due to Queen Elizabeth II's death.

I don't actually mind too much anymore though, it was just annoying when the episodes were available online but trakt.tv didn't recognise it as released.