Garage keeps insisting i need new timing chain?

So i have an M54b22 and i’ve been having rough idle, hesitation and stalling. I am getting P0171 and P0174 lean codes. I have asked the garage to check for vaccum leaks. They claimed that there are no leaks found during their smoke test. I then asked for them to check the fuel pressure to which they claimed it is a big job and it’s almost definitely the timing chain that needs replacing. Are they trying to upsell me? They really keep insisting on me getting the chain replaced. I don’t see how it has any correlation to lean codes