Laundering bribes, okay, makes sense.

There is little doubt that speaking fees are a form of corruption. But money laundering, in common parlance, is something else: ie. the money must first change hands, only after can it be laundered.

So if instead of receiving speakers fees, a politician is simply paid under the table, they could launder the money at a business they own by recording it under transactions that never actually occurred.

Speaking fees could be money laundering, if the politician is paid before hand and doesn't actually receive any speaking fees, but just presents a bill so both sides can make their books work out.

However since the services are rendered often years in advance of the payments, I would expect only a limited portion is dealt with in that manner. I expect politicians just trust the bankers, because if a banker ever stiffed them, then they would tell the other politicians about it and no one would want to work with that particular institution ever again.