Let me try to make this nice and simple for you
When I said "Chances are that the level of exertion making at-your-limit moves is considerably higher then diving for clams." It's because... the chances are that the level of exertion making at-your-limit moves is considerably higher then diving for clams.
Make sense? I hope that was clear enough for you.
Now, this is the part where reading comprehension comes in!
I didn't actually say the sentence "The higher the level of exertion, the shorter the period you can hold your breath for"!!!!
Instead I gave an example of an experiment you could conduct in the comfort of your very own home! By doing that experiment, or just by thinking about it, you could come to that conclusion on your own!
Now I know this is some high-level stuff- I mean like full-on high-school English - but I hope you can now understand, in hind site, having had it laid out for you in this "Heather Has Two Mommies" level breakdown, what I meant in my post.
In fact, let's take another look at that second part! What makes it even better then just asserting "The higher the level of exertion, the shorter the period you can hold your breath for", it let's you check that result experimentally!
And yes, I absolutely do think that hard bouldering, or hard crux sequences on steep ground, are much more physically intense then diving for clams. I also think that climbers train their lung capacity significantly less then clam divers do, and even if they WERE too buoyant, they would probably just wear a weight belt to get to neutral buoyancy.
As for you saying "Did not quite understand why you decided to bring weight into the picture." Well, when I said the word "weight" it was in the sentence "Yeah, I was kinda assuming that if you were diving for clams you wouldn't be carrying much body fat for your weight."
Now the important part of this sentence, and the word "weight" being used, is this part of the sentence: "body fat for your weight.". Now I could have said "you would have a low % body fat", but I didn't. Instead I worded it differently. Sometimes people do this, it's what makes some people (like yourself) very bad writers, and some people (like Sir Terry Pratchett) excellent writers.
The reason the phrase "for your weight" had to be tagged on at the end is because a very large person may have a low % body fat, but still have a higher quantity of body fat then a very small person who has a slightly higher % body fat.
Now, the % body fat is important in this case as it is the density of the individual that determines whether or not their buoyancy in water is greater then their weight. This, of course, is what determines if they float!
Anyway, I hope this made sense for you! Good attempt at making me back-peddle, though!