It is relevant if you want to interpret the meaning of the Constitutional phrase "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."The point a rich man may have once owned a canon is no more relevant today than the fact a T-Rex once existed in discussing today's dangerous weapons. If I supplied you with a canon from 1776 and Ranger with his weapon from his Army days, he will win 100 out of 100 times.
It would be very difficult to argue that the Founders/drafters of the Constitution intended the Second Amendment to prohibit the ownership of cannons and armed ships. To the contrary, they knew people who owned them and loaned/sold them to the government. Privateers were encouraged to get them some cannon - especially by stealing some from the Limeys. But there were no laws against such ownership.
Here is an inneresting article about "small arms" at Colonial Williamsburg, Virginia - and what happened when the Brits tried to steal them away and prevent their use against the King's percevied tyranny:
Magazine
www.slaveryandremembrance.org
If I were trying to ban arms, I might use this history to suggest that "mutual aid and protection" was the purpose of early gun thinkingh. But ... eventually, SCOTUS says what laws mean and whether they are Constitutional. That's the deal.