Here's the secret: it's total BS.
"Hustle" is almost always BS. And real hustle is a red flag. If people can't get what needs to get done within predefined work hours, it's either a reflection of incompetency or poor time management.
I worked at one of the ad holding companies mentioned for a few years, and the magic behind the curtain is that there's no magic behind the curtain.
Agencies by and large blow smoke up the clients' behinds as a matter of standard practice. And the clients typically know this (though I'm not sure they know the full extent of it), but put up with it because the work they are engaging an agency for makes more sense to outsource to the agencies, and since all agencies are posturing, they simply pick who they think BS'ed best (as if that's a reflection of what will actually get done).
When I was there, it was all about BSing digital competency and foresight. Everyone was trying to claim a better finger on the pulse of change. Now that change has somewhat slowed down (no major hardware upsets in the past few or next few years), it makes sense to start behaving more like a commoditized service and trying to brand oneself as offering greater efficiency than one's counterparts.
Which is why if I was still in the industry today, my pitch deck would probably include a slide about how you won't hear us talking about "hustle" because it should only be necessary if you're not optimizing your work day (and including out of country examples of functioning companies working even less than standard hours and doing great work, and numbers about productivity tanking as hours go up). "So you could pick a 'hustle' agency, but are they spending their 'productive' hours on you, or are they billing you for those extra hours operating at lower efficiency? If you see an agency claiming to 'hustle', personally we'd suggest hustling in the other direction ASAP."