I asked a similar question earlier, and DXV himself replied that he had not proposed this 80/70/48 split of the basic treasure cards as official for higher player counts. I remembered that I had read it, but my recollection assumed things not to be assumed. In 1st edition, the original Intrigue rulebook came with the following text: "To set up for 5 or 6 players, combine the Treasure cards from Dominion and Dominion: Intrigue". I.e. 120C/80S/60G. In 2nd edition, it says the same, but with other words: "Add all of the extra Coppers, Silvers, and Golds from Base Cards (or the other copy of Dominion) to those piles".
If I remember correctly: When producing the 2nd edition Big Box, they had room for some extra treasures, and DXV decided to put in the arbitrary amounts of 80C/70S/48G. This was no attempt by him to define the number of treasure cards needed for 5- og 6-player games. It was simply what he decided to put into the Big Box, and this was limited by the available space in the tray (and the fact that cards are shrink-wrapped in amounts of 100). The box says: "...plus extra base cards for 5-6 players". Unless I've missed something, there is no mention of actually using 80C/70S/48G anywhere, but I accidentally read the above-mentioned box text to mean "these are the amounts of treasure cards to use with 5 or 6 players". It simply states that they included some extra base cards, in case you want to play with 5 or 6 players.
When it comes to victory cards, curses and ruins, the numbers are clearly defined (and I'm not including the Estates normally included in the players' starting decks, as these are omitted when playing with shelters).
Provinces
2p: 8.
3p, 4p: 12.
5p: 15.
6p: 18.
ALL other victory cards:
2p: 8.
3p, 4p, 5p, 6p: 12.
Curses and ruins
2p: 10.
3p: 20.
4p: 30.
5p: 40.
6p: 50.
The number of potions and platinum cards aren't mentioned, so I imagine they should always be used in amounts of 16 and 12, respectively, even if you happen to have the Base Cards product.