Total cost of 2-3 toons?

[Wadair]

On Wiki it states:

1. Main refers all new accounts

Pros: Offers many granted levels to toons on your main account.
Cons: All 4 mounts can only be given to characters on your main account. You can't use your 70 main to boost.

Obviously that is slightly outdated, but why I couldn't boost on my main in this method? In the method below it seems to be possible:

2. Chain linking

X>A
A>B
B>C
C>D
Pros: Mounts on your first 4 accounts. You can use your main to boost. Main account gets mount. Levels are able to be granted to main.
Cons: ?

I can't figure out why couldn't I boost with method #1, or is word 'boost' used here for other purposes than 'boost your slaves with main'?