The point is when the dealer sells a loan - they get a commission. That is money the dealer sees in the transaction and hence has more space to negotiate the sales price lower. You're baiting them with money they think they will get.. so let them sell the loan, then close the loan. That was the point of the item mentioned.
Reality is most can't easily close out the loan - so in most cases, being armed with your own Financing is best to ensure you get the best possible rate. But go in with your interest rate, and 9/10 times, the dealer will work to have their banks match your rate. So they can still get their commission and the bank still gets the loan.
Dealers are incentivized to direct buyers to their partners. CA's point was - use that to your advantage because the dealer will see more items in their + column if you use their loan.