It's up to over 825k now apparently.
At present. Yes. Optus Sport is just a Soccer broadcaster. But that doesn't mean they'll only ever be that. We don't know what they have planned down the line and as a telco they much like their customer base also have a significantly greater commercial base to draw from.
One thing that is clear is that the major telcos are much larger businesses than our media companies and the premium ones (Telstra and Optus) are trying to transform themselves into being more than just a pipe service because that area is highly competitive.
They have money to spend and potentially care far less about how many new customers that spend creates in the short term at least.