Exactly. From most people, "smartphones are spying on us" sounds a little tinfoil hat. Coming from the Director of Military Intelligence, however, gives the message some credence.
This is probably a reasonable threat for executives to worry about, but it's still ultimately an expression of "we're concerned about a threat where breaking into a home and bugging the device or swapping it with a compromised one is a real concern".
This is closer to "if you can pay a team to go and find a way into that specific person there" then they're going to succeed.
But you're already close to "infiltrate the window washing company, and stick solar powered listening bugs to every window on the upper floor suites".
Pen-Testers use the 'cloned cleaners' pass' thing all the time and it works surprisingly well.
Most people have their email, personal photos, medical records, financial records, chats etc. on their phones. If you believe your phone is likely compromised, you should first move all of those off your phone before being concerned about realtime audio recording.
A year or so back I stayed over with some extended family, one of which was 'In to IT and earns a living from it. During that time (using the I.T guy's network) I noticed that one of my phones was eating through the battery at a crazy-stupid-mad rate. A quick check showed that this particular phone had begun uploading a metric shit-ton of data on an otherwise normally asymetric data profile (in other words Data Uploaded vs Data Downloaded Ratio).
A check with the 'family IT guy' confirmed that he had Remoted In to a high value clients' system to trouble shoot something at around the same time that one of my phones began to furiously upload any and every thing on its' memory card.
Sounds like one of those "Cool Story Bro" types stories right?
All I will say is that because one of my 'Low Hanging Fruit' thangs got p0wned I was able to alert the $corp of the problem (since the probe originated on their end not mine).
As with all things in Life, YMMV.
But most corporate networks likely have much more likely vulnerabilities that are worth much more thought than this, in my opinion. If your phone has been taken over to the extent that an audio/video stream can be recorded without your permission, I would bet also that much more valuable information than that audio/video stream can usually be obtained as well, for most people who are not used to high security.
People underestimate the spying potentially of mobile phones very often. They are always on, have a data connection, and run software that is abysmal for security.
Also, strict security practices that affect productivity often get worked around - for example, if I want to examine a diagnostics archive I got by mail on my phone, but the IT policy won't let me, I may well forward it to my personal mail, open that though Firefox, and get the archive this way. It's much worse for overall security, but pretty hard to prevent, and less important to me when I get a call about blocked builds at 10 o'clock.
Taking over a mobile device remotely and activating the camera and microphone is a real thing (citations available but https://www.wired.com/story/whatsapp-hack-phone-call-voip-bu... from 2019 provides a solid 'is possible to do' example).
I have worked in more than a few sites where all cell-phones needed to be placed in to faraday cage lockers located outside the highly secure areas prior to entering but laptops were ok to take in and use (subject to high contrast stickers over cameras, dummy jacks placed in mic sockets and wi-fi switched off).
Given that these were a mixture of national infrastructure, military and embassy type sites and was a thing ten+ years ago I think the various governments are well aware of their own abilities and assume that the opposition have equal capabilities.
I take your point regarding “Unless this is advice for state-level institutions discussing state secrets” but knowledge leaks and filters down in to the corporate world. Those same C-Suits that have to attend meetings discussing restricted information projects go away thinking – Huh, I wonder if it could be done to us by a competitor.
When you consider the difficulties of verifying the supply chain of a random mobile device (from the chipset through to all installed apps) it leaves so many security holes that it is best to assume that anything a nation state can do so can a well skilled hacker (or group).