Read everything written by Elmer Keith. You are not going to learn the answers to your questions on an internet forum. (you might but you're going to have to sort through an awful lot of nonsense.)
2 great thoughts cept for one problem; Keith and most of his writings are almost from another world when it comes to bullet technology, design and performance. Let's look at the .38sp. He (Keith) had a disdain for the .38 as a man stopper and points out some fine examples in his writings to justify his thoughts. But........... About all his examples were based off crappy RN ammo of the day. Let's face it, that's like comparing apples to oranges from what is available today. Same goes for the .357M. He found it acceptable for deer/bear using his Keith designed cast bullets but considered it inferior to his .44sp and .45c cast loads. No way was Mr. Keith ever gonna be convinced that a small bore with any bullet could match his big bores shooting HIS bullets and he may be right. But the gap can be closed considerably and unfortunately he never saw the wide range of bullets we have today. HP's back then were nothing more than varmint bullets in construction. Don't get me wrong, I love readin Keith and he was the Guru. He just didn't have all the information required to judge what is good or great today since well, he didn't live to see today. Again, unfortunately. No matter, Keith is a must read.
OP, someone either spouted a general rule as gospel or you took it as gospel. Either way what you understand is nothing more than that, a general rule. Not all cast are created equal and not all HP's are created equal.
Take a good HP like Hornady's .357 180gr XTP; while it may not penetrate as far as a hardcast bullet of quality design and construction it will have zero issues going thru deer or other BG and offers a greatly expanded wound channel compared to hardcast bullets. Heavy for caliber HP's of good construction definitely have a place in the hunting world.