I also suspect that a lot of the people who are eating out in restaurants all the time are on planes a lot and at late business dinners, many of which are determined by "safe" customer choices like steakhouses. (Though, without further info, I also suspect to it defaulting to fast food on an absolute basis.)
Also: truckers (largest single occupational category in many classifications), cabbies, trades workers, etc., etc. If you're in a captive market, whether that's airports, convention centres, motorway service areas, lunch wagon, lunchroom, etc., your choices are going to be limited.
On business travellers vs. truckers, I'm finding ~ 3.5 million truckers in the US, who all but certainly eat on the road at least one meal per day (local) and more likely 3+ (long-haul). <https://schneiderjobs.com/blog/truck-drivers-in-usa>.
For corporate travel: there are ~400 million long-distance business trips annually <https://www.trondent.com/business-travel-statistics/>. Trips-per-person is harder to find, though one source gives 6.8 trips/year, which gives 60 million travellers/year. So that's more than truckers ... but it's a lot fewer trips (and meals). I'd put money on there being more trucker meals-out than business travelers'.
Fleshing out further: we really want trips per year, for each classification. I'm going to assume truckers are on the road 250 days/year (roughly 5 days/week) ... we'll do variance after in case I'm wrong. That gives 3.5 million * 250 or 875 million trucker trips/year, more than double the business air travel number. We could cut trucker travel in half and still be somewhat above the business air travel trips figure.
Whatever the exact number, it's probably safe to say that directionally way more "meals out" are grabbing some variety of fast food than some variety of upscale/fine dining. Not all that fast-ish food is unhealthy/bad but a lot is as a steady diet. Which just reinforces the point that drawing a broad brush eating out will kill you doesn't have a lot of support. Eating at a nice restaurant once a month is almost certainly not a killer.
The truckers vs. business travellers comparison was mostly me just trying to suss out what the relative magnitudes of those were. Information's sketchy, but inferences can be drawn. That was independent of your points, which are valid and insightful.
And yeah, the idea that 1) most "restaurant" meals are fast-food franchises and 2) that's not especially healthy on a consistent basis, as well as that 3) specific choices about menu items can have a major impact are ... fairly self-evident. Pity the study doesn't seem to address those, at least based on the unembargoed bits at the shared link above.
