No, you need a linked brokerage account to get the fee waiver. You can keep the CPP account, pay the monthly fees and get the AMEX reimbursed. Or you can open a different sort of separate brokerage account, keep $25k+ in the CPP plus keep whatever is needed in the other brokerage and then get both the fee waiver and the AMEX engagement bonus--this is the so-called 'free' AMEX Plat.
I have a regular AAA brokerage. It's OK, the annual fee is $150 (which I haven't been charged yet) and there aren't any other fees if you don't trade, commissions from $6 to $27.95 if you do. Just set up a low-cost simple portfolio (ITOT + VCIT for example), set up dividend reinvestment and let it ride for a decade or two. If your AI account had their low-cost balanced or growth portfolio (all iShares, Vanguard or State Street SPDR funds) then you can just transfer that into a regular AAA and set up dividend reinvestment and you're done, no commissions.
Commissions can be anywhere from those levels into the hundreds, or even thousands for a very large trade(s). I just don't want folks thinking that MS commissions are somehow capped at those levels. Not even close.
For a full-service brokerage account, if you don't have a managed account (fee-based, no commissions), this is how you pay the advisor.