How is that true? These companies have shown that you can scale well if you don't have monolithic approaches, but that doesn't mean that you cannot scale if you have one. Just because someone is successful by using approach B doesn't make approach A unsuitable.
Quite the opposite. Some banks handle more than a billion transactions per day on monolithic infrastructure (think of big investment banks with a sizeable amount of trades per day). It's very costly, but it does work reliably.
EDIT: One thing I don't get: Most banks use a central database somewhere because you need to have accounting values that you can rely on (e.g. how much money you have on your account). How do they do that? They must use some synchronization with locking that works across all services which rely on them, but how will that scale well when they get dozens of transactions per second? How do they guarantee the integrity of their data?