As the ASA ruling describes, the card was headed "SORRY, Your current broadband provider couldn't deliver." Alongside some seemingly handwritten info such as date and time (like you'd see on a courier's card), the list of undelivered items included movies, games, music and cat videos. Cute, but perhaps a little too clever on Hyperoptic's part, as one recipient complained to the ASA it wasn't immediately apparent the correspondence was an advert.
Despite the flipside of the card (and the envelope it was delivered in) dropping various hints by way of logos, promotional terms and various mentions of broadband, the ASA agreed it could give a confusing first impression. That was almost certainly the whole point of the campaign, of course, but a breach of advertising code is a breach of advertising code. Typical of these rulings, Hyperoptic is escaping with only a light slap on the wrist. The ASA has simply told the ISP it can't use the ad again without appropriate tweaks, and to not be so naughty in the future.