That article does not mention anything about Tandem releasing an API, which is what Monzo, Truelayer or anyone other aggregator would need to connect to your Tandem account. Truelayer won’t help them with this. The first thing to establish (by asking Tandem) is if/when they will be releasing an API. Once that’s been released, then Trulayer and Monzo can be asked if/when they will be supporting it.
The partnership described in the article only relates to providing Tandem with the ability to display 3rd party accounts in the Tandem app.