Megaton warheads sound scary and look cool, but are limited in utility. A nuclear bomb emits a bunch of heat and radiation from a point and it expands in a sphere - in all directions including up and down. The bigger the bomb, the bigger that sphere is. That means with a multi-megaton device, you're wasting huge amounts of energy making a fireball that reaches up into the stratosphere where there are no targets. It's a way better use of materials to have multiple warheads in the 100kT range and keep all the energy on the ground where your targets are. Given the weight involved (a 40mt warhead would probably be at least 10,000kg) I don't think they're lifting 16 of them. They probably mean 16 ~500kt warheads for normal use, or one 40mt warhead for the international rooster measuring contest.
MIRV warheads are not new. The US has 14 ohio class submarines, each equipped with 24 trident missiles. Each trident missile can carry 14 warheads with a yield of 475kt each. Fat Man and Little Boy were 21kt and 15kt - 475kt will obliterate any city on earth. Each warhead can be independently targeted, so assuming no anti-missile defenses 336 cities can be destroyed. A single ohio class submarine can end civilization on an entire continent. This capability has existed for over 40 years.
I haven't researched russian capabilities as much but I'd imagine it's been similar.
