Hmm i'm sure there is some good math behind those claims but logically no multicolored deck will ever have 18 sources of one color and in a tri-color you will probably not get 13 either unless you have a dominant color and splashes. That's why you need mana fixers so you can search for the colors relevant to your current hand, or lots of draw.
I guess the point is that tri-color aggro decks will very rarely work as they cannot curve out in a reliable way so they need ways to delay while the land falls into place and in an acid moss meta that is suicide.
Actually I'm pretty sure Babs took the '18' sources directly from
Dr. Karsten's article.
His tables, when taking into account a few premises (read to understand better) show you need 18 sources for double colours on t4 if you want to curve that card in 90 % of cases. She neglects to add though, that fetchlands count for both colours, and cheap cantrips count for 1/2 to 1/3 to 1/4 th coloured source (depending on quality). Add to that the
free mulligan in duels and the fact that there are
way less 4 offs of the dual coloured cards in question, and the end result is that the 18 is a bit exagerated.
I try to hit AT LEAST 13/14 in tricolour decks when I include Rogues or the like; but if a deck
really leans on certain cards, 16 should be attainable in most cases in Duels, but mainly in dual colour decks (those numbers include cantrips and vines). I've done 17/18 but all situations involved
Consul's Lieutenant and / or
Knight of the White Orchid in azorius tempo or control. You wanna hit those things t2
Example : 4 colour Spectrum mill deck with +- 16 Blue (cantrips online, t4 double costs, t5 double costs, t8 triple cost) , 15 Green (manafixing and cantrips online, t5 double costs), 14 black (for effectively only 6 cards) and 9 red (probably need to add one/ 2 for a green and black) for 5 cards :
http://forum.nogoblinsallowed.com/viewtopic.php?p=418806#p418806