He's both right and wrong - the increase in the stockpile numbers is about swapping over to the new warhead, but it's also about increasing the number of warheads that are deployed on each sub. not to the kind of numbers the subs could sail with, but a higher number than the 40 or so of the Cameron era.
The decoys are common knowledge, if your enemy has 100 anti-ballistic missiles (missiles?) and you present him with 100 targets, and he has no way of determining which are 50 warheads and which 50 are decoys, some warheads will get through, and he knows that.
The 'renting' idea is both right and wrong - we are dependent on the US for the missile bodies, we bought into a pool of missiles, and we can have 55(?) at any one time out the wider USN/RN pool, and they are serviced and upgraded by the US in the US, but the warheads themselves are designed and built in the UK, even if they do end up bareing more than a passing resemblance to the US warheads.
However, the wider renting thing applies to all military hardware - we see it in the defence review: unless you (fairly) constantly apply money to your gear for upgrades and refurbishment it quickly becomes obsolete. In the 15 years of Iraq and Afghanistan we effectively stopped spending money on the upgrade paths for anything that didn't get used in the desert, so stuff that should have happily been able to carry on in service for years (decades?) is now going to be turned into razor blades because it is now so behind that no upgrade path is either available, or so expensive as to be impractical. Warrior, AS90, E-3D AWACS, and many others are going for this reason.