Agree, no point in referencing a dozen places to buy XMR in theory, where in practice you can use only 1 exchange (of course P2P can be a very good way too but we are talking exchanges).
No. In practice you can use any of the dozen+ places to buy XMR. The other options aren't less viable just because Poloniex is the preference.
In which of the exchanges mentioned above can someone buy 10k xmr without slippage?
That's 'in practice'.
You can be a sophist and play with words as much as you want but someone has to wonder why...
EDIT:
That doesn't change the obvious facts.
Put there offer to buy 10k XMR and sooner or latter someone will fill your order.