All that letter means is that you would be reasonable to sell it off or keep it or do whatever you want with it without the police coming back later and coming after you for theft or selling stolen goods.
And the police's good faith opinion doesn't really come into play though as Activation Lock is governed by Apple's own privacy policies and such.
Think of it this way: if person A sells person B their iPhone and forgets to unlock it/disable Activation Lock. And person A included a signed and notarized Bill of Sale to person B. Person B tries to get that iPhone Activation Lock disabled, and Apple is still going to tell person B that only person A can do that; and that Apple won't do it unless Person B has a copy of the original retailer/authorized reseller receipt for the original retail sale.
Nothing that Person A can give (notarized bill of sale, etc.) short of a copy of the authorized retailer receipt is going to enable Apple to do anything per their own policies. Person A, in this case, is unknown and not available. Which means there's nothing you can do.
About the only exception that I know of is a death certificate and legal executor paperwork or a court order. The letter from the police releasing the found goods to their finder (you) after the statutory holding period in your area (sounds like 6 months, which is typical) doesn't change any of that.
It's a paperweight like the other fellow said, or a source for spare parts, and that's about it. You did the right thing by turning it in, but other than for spare parts it's not going to be of any other benefit to you now that you've gotten it back. Sorry my friend!
