Part nonsense part not.
I have doubts that Chevy ever used nickel in any production blocks regardless of magazine claims otherwise.
Thin wall casting mostly uses less material which saves both money and weight, I'm sure the OEMs worry about it in that order.
The factory 4 bolt main has more to do with spreading the forces over more of the web, the cap stability really comes from that not so much about more clamping force on the cap although that does happen. The aftermarket 4 bolt caps angle the outer bolts into the pan rail where there is more meat to bolt into. The help this brings is now the bulkhead is being somewhat supported by the cap instead of just supporting the cap as in the factory design as some of the loading is being transferred into the pan rail area beyond the bulkhead.
A good balance job and damper does a lot to relieve these center caps of having to deal with loads that would like to put the center of journal rotation somewhere besides the center line of the crank.
You will note that the GEN III and later SBC's use a main cap in girdle arraingement that was also found on the Allison aircraft engines of WW II. The other popular design used by Rolls Royce/Packard and many German and Russian engines of the era was cross bolted mains. The effort not only to stiffen the bottom end but to distribute the crank loads over a greater area of the block.