Earlier quoted context omitted.
If they were obtained without a warrant, wouldn't they be inadmissible as evidence, especially if they were being used in an inculpatory rather than exculpatory manner?
The government lawyers' opinion is that they can later get a warrant to access the data, long after it was recorded (see the whole discussion on "collect" meaning to take a book off the shelf and read it). They also seem to think it's ok to datamine the recordings as long as it's done anonymously.
He probably was told in his classified briefing that NSA could record and collect phone calls without a warrant.
Mueller, on the other hand, would be talking about actually tapping into that data, which would need a warrant.
I'm not sure I buy the argument that you can record a phone call without a warrant just because it's using VoIP or some equivalent instead of a physical wiretap, but it does sound similar to the other arguments that have been made to collect data for later analysis by warrant if necessary.