I think, and this is pure opinion, that the reason for the penetration preference by both of these men was the poor quality of soft point bullets available to them. There were a lot of bullet failures on African game when the move was made to velocities beyond 2,400 fps. These men, in light of the bullet failures and general poor performance, relied on penetration arguments as their alternative. They had no other way to go at that time.
Today, we have bullets from several companies that provide both penetration and proper expansion at higher velocities... but the possible solution still remains that many of the older rounds at moderate velocities get the job done very well. I've come to the opinion that our current emphasis on high velocity projectiles actually works against finding a good balanced load for a wide variety of conditions and game size. Many of the turn of the century rounds continue to provide more consistent performance than newer high velocity versions.
Writers stress very small long range trajectory advantages (many of which are impractical for most hunting conditions and ranges) and completely ignore how the round penetrates and delivers trauma.
I also think that this is why the Garret (SP?) .45-70 hard cast loads have performed so impressively for some hunters in Africa and it has given the .45-70 the opportunity to even be considered as an African big game rifle. Also I have read and heard of cases where the heavy .30-06 loads have outpenetrated the .300 Weatherby Magnum on African game. I believe this is for much the same reason. I have a .300 Weatheby Magnum and use it often on North American game as well as .45-70 and .30-06 rifles... but I agree with you that there are much better rounds for African game than any of these including those you mention.