If it's not in the lease agreement, then you did not agree to it. Even in the absence of the law prohibiting the late fee, the landlord would not be able to impose it because it is not in the lease.
It is of course up to you whether you want to test it. It may be more trouble than it is worth. Then again, it's probably not worth the landlord's trouble to begin eviction proceedings over a late fee.
Paying late and refusing to pay the fee would probably sour your relationship with the landlord, which is usually something you want to avoid. If you do pay late at some point, you may want to point out to the landlord the relevant provision of Massachusetts law. The matter would probably end there: either the landlord doesn't know about it, or the landlord is hoping that you don't know about it. Pointing it out in a polite manner will inform the landlord of the law and that you are acquainted with the law. Unless the landlord is quite unreasonable, that should take care of it.
Whether there is any law prohibiting landlords from announcing an intention to take a prohibited action is indeed a different question.
Is there any legal reason for me to bring this up and get the landlord to formally acknowledge that they will not in fact charge illegal late fees?
No. You have a contract with the landlord that already says you are going to move in. He cannot modify that contract simply by sending you a letter.
