before i saw your 'hand waving' sentence i was thinking "some of this doesnt make any sense"
however, i think it's implicit that much of what is making their economics work is 1. counting on making the project work longer - partially because of less hardware & 2. negotiation of long-term contracts at very favorable current rates (this is why i don't think the major service companies will recover as quickly as the E&Ps and lower level suppliers).
I'm not sure what the 5% price realization to Brent is but Tengiz oil is very high sulfur so I'd expect a steep discount to Brent. The kinda good news i suppose is that i think it's light oil (~45 API). Again, that wont fair well against Brent but it's not like high-sulfur Venezuelan or Canadian crudes.