That's referring to the time that the scammer needs if the recipient falls for their bait and initiates contact.

You only want the truly gullible to send that first email, or it would be a waste of time for the scammer to talk to all the people who wouldn't wire transfer their money a few days/weeks later.

Right, all contact after the first email has to be tailored to their responses. Even gullible marks usually need hours or days (at minimum) of building rapport before they're actually comfortable enough to be conned into executing a transaction. If conning people into directly handing you cash were automatable like phishing, you'd see a lot more con artists and a lot less of other crimes.

After reading your reply the whole paper made sense to me, thank you.

The initial contact is 'en masse', but the followups are all by hand. Time spent by the scammer to respond to potential marks is, in fact, a scarce resource.