Showing a fit - splinter bids

The conventional wisdom states that if you show a splinter and then ask for key cards yourself, that keycard ask is exclusion keycard and of course your short suit is a void.  The reason behind this is that a splinter should be a limited bid and that if partner signs off in game, you now show extra good news (it might not actually be all that good for partner) with the void.

I've never really liked this conventional wisdom but now I think I see the reason.  A problem arises if partner takes charge and bids a small slam.  If you have a really good hand, you know that a grand will probably make but without a void in your short suit, you can't override partner's decision in case there is a loser there.

Here's an example of a hand that is getting close to this maximum strength for a splinter.  Sitting South with the GIBs on BBO, I held ♠AQJ9 ♥K643 ♦3 ♣K542.  My GIB dealt and opened 1♥.  With no particular understanding with Mr. GIB, I simply bid 4♦, which seemed the obvious bid.  My GIB took charge and, after finding that I had two key cards and the ♣K, put us in a grand slam, taking 13 tricks and as many imps.  It actually wasn't the greatest grand slam and indeed only one other GIB even took 13 tricks.  Given that my GIB had ♠2 ♥AQ9852 ♦AQ64 ♣A6, where the ♦Q is worthless, he might have been slightly more conservative.  OTOH, he has a magnificent hand outside the diamond suit and he can almost count 13 tricks.  Ruffing that fourth diamond might be tricky.

No other pair reached 7 and only half of the rest even bid a small slam.  I thought I'd look to see which other players (all human) with my hand had bid 2NT and which had splintered to see if the splinter was the key to bidding the grand.  The answer absolutely astounded me!

No other South splintered!  Only two bid 2NT!  What other bid is even possible?  Well, here goes: four votes for 2♣ (including one where RHO had overcalled 1♠);  four votes for 1♠; two for 4♥; one for 3♥; and, unbelievably, two for 2♥!

I realize that many of the people who play with the GIBs probably can't get good partners of their own, and it's quite likely that several of the humans don't know that GIB plays 2/1.  But this demonstrates to me that the ignorance of people regarding how to show a fit for partner is unbounded.