It's not a contactless payment, it's a bank transfer. The person who found the wallet sent payments of £0.01 to the owner. When you make bank transfers you can include a short payment reference which can be any text you want, and it appears on the other person's statement.
The UK has the Faster Payments Service which enables most bank transfers to be completed in a few seconds, so they would have seen the messages immediately as soon as they logged into their bank account to freeze the lost card.
It's not so strange. Many debit cards in the UK have the sort code and account number printed on them, which is all the information you need to send money to that account.
When I send a payment I can either include a structured message (a code in the format xx-xxx-xxxx) or a "free text" message in which I could easily write "found your wallet contact me on abc@email.com"
If not mistaken this was done via contactless payment, right? how is that possible to add a message? (maybe you can in some country)