Indeed, that's what it means - composing precisely (or as precisely as possible) at the time of shooting, rather than composing loosely and "fixing" composition in post-processing.
To me, composing in camera is the way to go; however, there is one important caveat - image ratio. When we choose a camera system, we also buy into an image ratio, given to us by the manufacturer. I.e., digital medium format uses 4:3; so-called full-frame 35mm uses 3:2; film medium format used 1:1, 6:7, 4:3 and a variety of other formats, old tech cameras used 5:4; and so on.
The problem with that is the world, unfortunately, doesn't always "fit" into what image ratio our manufacturer of choice imposed upon us. E.g., my cameras for the last few years (and for most of my career) have featured 4:3 as image ratio, but for landscape-oriented images I often go for wider than that (rarely 3:2, more often 16:9, rarely 2:1), while for portrait-oriented images I often go for 5:4. I also like to use the square format, when the subject matter allows. Therefore, I am forced to crop.
So, perhaps it is good to split cropping in two:
1. Cropping for "artistic" reasons, e.g. to solve the image ratio problem;
2. Cropping to "save" a badly composed image;
The former is fine by me, in fact I encourage it when needed to strengthen compositions; the latter I try and not do.
As well, there are a couple of instances of cropping we need to consider, when needed:
1. The cropping caused by straightening titled horizons, and by straightening converging verticals in post-processing;
2. The cropping caused by compositional issues that couldn't be solved in the field, or constrains that couldn't be dealt with at the time of shooting, i.e. having prime lenses and no room to move around in order to avoid including something, and the like;
In these cases, cropping is just a necessity.
Finally, if you use a very high megapixel camera (such as a Phase One 150 Mp, or Fuji 100 Mp), you can consider the use of cropping rather than carrying extra lenses: skip a lens, make your bag lighter, and crop to gain "reach", so to speak while still having enough Mp for a good resolution image.
One anecdotical thing: Cartier-Bresson was famous for not cropping, and he said
"If you start cutting or cropping a good photograph, it means death to the geometrically correct interplay of proportions. Besides, it very rarely happens that a photograph which was feebly composed can be saved by reconstruction of its composition under the darkroom’s enlarger; the integrity of vision is no longer there.”
– Henri Cartier-Bresson"
However, what is probably his most famous image - Behind Gare St. Lazare (the jumping man over a pool of water) is heavily cropped
Clearly, he couldn't compose it as he would have wanted to, but the image was so strong that he decided to "save" it by cropping the negative. Erwitt cropped, the famous image of a woman's feet + dog is cropped; and so on.
So, I wouldn't get too "religious" on the "composing in camera" thing
Whatever works for you, and whatever results in a strong, powerful image, while keeping the "technical" side (final resolution, and so on) good enough, is fine by me. The biggest reason I recommend "composing in camera" is to help people develop their eye, their shooting discipline. The occasional crop, I am perfectly fine with.
Just my .02, of course. Best regards,
Vieri