the engine WILL be electrically connected to the frame. In fact, it would be difficult to insulate it from the frame.
Actually, it's quite easy in practice - well-painted or -powder-coated frame, carefully-installed engine, the engine is often insulated from the frame. I wasn't the first to advise a wire from the engine; I copied the tip from John Healy, working on 1970's Britbikes electrics, I noticed Lucas did it, and there were random electrical problems when the wire between engine and battery +ve was missing.
In order to avoid having many wires on the battery (+) terminal, or having many inline splices or connectors, it's convenient to have a "ground post" - where? Well, on the frame of course.
Uh-uh, no "of course"; OIF and 1970's triples (? Triumph), Lucas soldered the ends of component Red wires to a washer, that washer attached to an engine component. If the wire from the washer to battery +ve was forgotten, because other component return wires are by definition attached to the engine exacerbates the aforementioned random electrical problems.
Careful assembly, whether or not any non-electric cycle part is connected is irrelevant.
Also, I've never used a "ground post", frame or engine. The vast majority of electrical components are either at the front of the bike or in the middle, the latter including the battery; I did my first rewire long before there was an internet; I found connecting the components around the headlamp to one snap connector, the components under the seat to another snap connector and connecting the snap connectors together with one wire used the least wire, far less even than Lucas used connecting components to a "ground post".
And to the frame are connected many other metal parts, fenders, fork yokes, handlebar, etc. In fact it's necessary for the handlebar to be "grounded" if you have a one-wire horn switch.
So, like it or not, the frame and most of the metal parts on the bike will be electrically connected to "battery return",
on that fails in reality. The horn doesn't work because, while the horn button might be "grounded" to the handlebar and the handlebar to the yoke, the forks can be insulated from the frame by something as simple as well-greased steering bearings ...
Simple and ime long-term reliable is to connect a wire from the horn button or the handlebars into an existing Red wires' snap connector inside the headlamp shell or under the tank.
The tail/stop bulb is another component that relied for many years on metal-to-metal contact between several random cycle parts, and fails either when dirt and corrosion get between the metal components or better-painted or -powder-coated components are fitted. Lucas eventually 'admitted defeat' by fitting rear lamps with the third, return, wire from about mid-'71-ish afaict.
Otoh, Lucas never fitted their turn signals with return wires; old-Britbike internet forums are littered with posts both by people who bought Triumphs and BSA's new and removed the turn signals soon after because they stopped working, and/or they can't get them to work now; fixing that by adding return wires was so common long before there was an internet that I do it and advise it as a matter of course.
Unlike Lucas, the Japanese bike manufacturers did rely on "most of the metal parts on the bike will be electrically connected to "battery return""; again long before there was an internet, adding a Lucas-style supply wires
network connected to battery -ve solved so many apparently-random electrical problems on high-mileage Japanese bikes it's something I'd do as matter of course.
It's also convenient to use the term "ground" (or "earth" if you prefer),
Like any jargon word, it's fine if everyone in the conversation has exactly the same understanding of the jargon. Unfortunately, reality shows different people don't of "ground"/"earth" - e.g. recently, the OP of the "Digital Speedometer Problem" thread on British Motorcycles In General? While there are many, many similar examples, another I remember particularly was on TriumphRat - a contributor posted he'd connected his 'negative ground' truck to his 'positive ground' Triumph ... luckily, his Triumph's battery was shagged so he only put about 17 or 18 Volts across his Triumph's electrics, which luckily didn't have any electronic components ...
That's the reason I try to avoid using "ground"/"earth", nothing to do with sub-atomic particles, "supply" and "return" are a convenient hook and, more importantly, aren't jargon.
We know that "ground" means a return to the battery;
BSA's, yes; Triumphs, not always ...