One in three, multiplied by the collation of rares/mythics to commons/uncommons gives you 5.3%
That volume cuts both ways, the legend density gives Commander builders years of material, but it also means a lot of cards competing for the same binder pages
With toxic, however, the amount of poison counters that can be dished out is much smaller and can't scale up, thus requiring a higher number of relevant creatures, and frankly they just aren't here
Play both decks against each other, then combine them to create one bigger deck
While I cant be certain of the latter, I can say casting Catastrophe for land, or any other Land Destruction spell, comes with huge cost and risk for the player playing it, too
Basically if the original card is legal, the promo is also okay to be played in that format