Yes, most of the cruise lines send out these letters, they also say you can not leave the ship unless you are either booked with them for shore excursions or have a Russian Visa in your passport.
The letter goes on to state a few other dozen flat out lies and I am amazed no one has sued for commercial fraud. There are sent by their marketing department and I am sure their legal department has not approved them. It might qualify for a class action since millions of dollars from hundreds of thousands passengers is involved.
The truth is that you do NOT need a visa to leave the ship, while in port the only ones who can determine whether you can enter a country is the immigration officials of that country, not ship marketing policies.
In Russia immigration officials allow entry with either a visa or proof of being booked on a sponsoring, authorized tour company's tour. The visa waiver for St Petersburg has been on the law books since well before the existence of the Russian Federation, it was transfered from the old Intourist days of travel to Russia during the Soviet Union.
To address your question, as soon as the ship is delcared to be "Clear" of customs, you are invited to come ashore to present your documents to passport control. When inside the waters of a country, the laws of that country trump those the polices of the ship so if you are invited by the controlling authority, no crew member can stop you from leaving. That crew member or ship officer can be arrested for illegal detainment if they do. In reality the ship will not try to stop you, they know the laws better than the marketing department which likely have never been within 5,000 miles of Russia. Their intent is to intimidate you into buying their overpriced crowded group tours and making you think there are no options. But after you actually do book somewhere else, they do not try to carry through with their intimidation. A few ships announce over the PA system that all non-ship sold tour members must assemble in a lounge far away from the gangway until their departure is announced. Of course their departure is never announced so many have missed their tour. One even posted a guard to prevent anyone from leaving the lounge. They did get sued and the verdict news had the legal department of all carriers revise their ship board rules but the marketing departments apparently ignore the rules within their own company.
Be ready to disembark as soon as the ship is cleared because there will be a big crush of people wanting to be the first off the ship. The ship arrives at pier 33 at 6:30 and clears by 7:15 most often. be off in that first group or else you might be behind a mass of 2000 people who were intimidated into buying the Princess cattle-call style overpriced tours.
_________________
Consult your friend on all things, especially on those which respect yourself. His counsel may then be useful where your own sel