Only valid during the two summer months. It's a rather weak simile of the German Deutschlandticket (now 58 euro/month but valid all day on bus/tram/metro and local/regional trains, but not on long distance trains, in a much larger country).
It's been 63 euro for some months now.
It started out as an idea to introduce the same concept as the Deutschlandticket in NL. But the government has a budget deficit and the national railway company expected a capacity issue during peak hours. As a result the ticket is only valid in off-peak hours and the low price is only for 2 months.