Yes, that is correct. If you want to hand deliver a note to your nextdoor neighbor, their mailbox is off limits. The federal government views that mailbox as some sort of exclusive domain of the USPS. You can leave the note anywhere, other than their mailbox.
The text of the law is:
>Whoever knowingly and willfully deposits any mailable matter such as statements of accounts, circulars, sale bills, or other like matter, on which no postage has been paid, in any letter box established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any mail route with intent to avoid payment of lawful postage thereon, shall for each such offense be fined under this title.[1]
[1] https://www.law.cornell.edu/uscode/text/18/1725
(An interesting possibility would be to affix the correct USPS stamp on the note, void said stamp by crossing it out, then hand delivering the note into someone's mailbox. In that case the correct postage would have been paid, so maybe that might be legal? Not a lawyer, just pondering an interesting possibility out loud.)
That all sounds commercial, not dropping a note to a neighbour.