We'll take it with a pinch of salt for now, seeing as the dates given are August 9th to 11th, and we're supposed to be playing Norwich on 7th and Barnet on 11th (see pre-season fixture list).

Can anyone shed any light?

In reply to the above...

Julio Romo points out that it was just one website, Libredirecto.com, which suggested (first on 25th June) that Arsenal would be taking part in the tournament. Other sources (eg here) agree that Real Madrid and Deportivo (the hosts) are so far confirmed participants. Sporting Lisbon and Penarol (of Montevideo) are understood to be in negotiations with regard to making up the 4. It looks like Arsenal may have been invited too, but it seems unlikely. Having said that, ANR, who often claim inside info, are saying that Arsenal are taking part (in an article well worth reading for anyone who wants to believe that Vieira will be staying at Arsenal!).

Dan Rickard points out that the CalcioMercato story just says "Arsenal". Perhaps they're talking about one of the other clubs with that name.