But since Russia has a vast nuclear fleet and the diesel subs stay closer to shore and I'm guessing AIP tech was not seen as necessary.
Being quiet and a technology with a lot of applications in a lot of areas it is probably seen as very useful and desirable, but as you point out they have nukes which is the ultimate AIP, if a bit expensive and a little noisy in comparison to an all electric drive system.
When the Russian AIP is fully operational the size of the diesels can be dramatically reduced to emergency energy supply so it wont be exactly the same as having three propulsion systems.
The idea with the Russian AIP is that the hydrogen is extracted directly from hydrocarbons but oxygen is also needed for the process where hydrogen is fed from one side of the system which oxidises with oxygen to form heat and electricity and the byproduct is water plus solid carbon.
In comparison the same fuel and oxygen going through the diesel engines also produces heat and water but it also produces carbon monoxide which is highly toxic and bad for the environment. In comparison solid carbon can be dumped safely with no risk to the planet or animals, though you could also store it and use the carbon to make useful products too.
The advantage of the Russian system is that all the infrastructure for supplying ships with diesel fuel is already present in almost every port on the planet and liquid oxygen can be produced at the pier with an electricity supply plus water.
For something more exotic like an airship you could use diesel enigines as the backup electricity supply, solar panels... taking advantage of its enormous surface area, so you would need batteries to store solar power as it is collected. The propulsion itself could be electric motors with propellers in propulsion pods for manouverability. The structure could be super light and super strong and super fire resistent carbon fibres and composites and the ballast could be water that the fuel cell could convert to lifting hydrogen when lift was needed, or water ballast together with heat when that was required meaning venting would not be needed as much which makes operation cheaper because it is more of a closed cycle and more efficient.
Sadly the sight of the Hindenberg burning has coloured peoples view of airships as being dangerous... they are when you make them out of the stuff we make smokeless gunpowder out of... the skin of the hindenberg was combustible with an inside structure that consisted of bags filled with flammable hydrogen with pockets of air in between... the worst possible mix of fuel and oxygen, wrapped in a gunpowder skin. Replace the gunpowder skin with carbon fibre and nomex and composites that don't burn, and fill the gaps between the hydrogen bags with nitrogen and you could put a dozen burning road flares inside the thing and nothing would happen except some scorch marks. Hydrogen needs oxygen to burn. When it does burn it burns with an invisible flame.
There are a huge number of reasons to continue to develop the Russian hydrogen fuel cell technology, but so that it can be fitted to a sub is probably not one of them.
Improved fuel cell technology and high speed charging technology probably make fuel cell and other air independent power supplies redundant... except nukes of course...
A small 1 metre by 1 metre by 4 or 5 metre long nuclear battery like those used for their new laser systems (truck or aircraft mounted) could be something, but they were also toying with sea bed nuclear power stations where Russia subs could moor to these underwater NPP presumably to recharge mid mission for perhaps half an hour to an hour and then continue on fully charged without making a lot of noise and not needing diesels or AIPs making their subs much much cheaper... you could have active defences around your charging stations like torpedo nets and even mines and other protections to prevent access by enemy subs or to prevent the sites being easily attacked.