They are saying the PM is responsible for collecting information from the customer and bringing that information to the team in an efficient manner so the team can make decisions based on that information.

If the PM brings incorrect or inadequate information, the resulting product will be poor because the team will have made its decisions on the basis of that poor information.

It should not, but the PM is accountable. GP put it nicely, effective PMs know better what customers want than customers themselves. This keeps iterations fast as PMs can immediately answer questions on trade offs instead of going back to the customers (who don’t know).

The PO is an always available customer.

And here's the wonderful thing about agile - if you happen to have an always available customer, you don't need a product owner