It was correct because it was obviously the entering into a contract part I was referring to. If the company delivers the goods then they have agreed to abide by the contract even if it was a mistake.
If this ticket is valid via the Cumbrian Coast then perhaps you could explain why trainsplit doesn’t offer it Mon-Sat, only Sun? If the only restriction is you have to travel on a Northern train then it would offer it 7 days per week.
The average consumer would have no idea what "DalesRail" is, and would therefore have no idea what significance (if any) the term "DalesRail Off-Peak Day Return" has, as compared to an ordinary Off-Peak Day Return.
Accordingly the consumer is unlikely to be aware of there being any mistake (where, by contrast, pricing a TV at £10 is obviously a mistake). Without that knowledge, the circumstances where a contract can be undone are much more limited.
More to the point, there isn't any mechanism for train companies to "unsell" already sold tickets. So whilst they might have the legal right to do so in a narrow set of circumstances, it's just not something they can do in practice.
Trainsplit sells any ticket type that hasn't been disabled for fulfilment by online retailers. Most other retailers work on the opposite basis - they only sell tickets within a defined list of ticket types (e.g. SDR, CDR). This is why Trainsplit will sometimes be the only retailer to sell a given ticket.
In this instance, the fares data limits DalesRail CDRs to being sold on Sundays - similar to how Thameslink-priced CBA/CBB Super Off-Peak Day Returns are only available on weekends (and bank holidays).
Given the above, there is nothing surprising about Trainsplit being one of few retailers to sell this ticket online, or about it only doing so on Sundays.