No, these are not guaranteed numbers, this is all based on the split that should be rewarded with the assumption that 4,000 Obelisks are in use. If the number of Obelisks in use goes up, then the expected rewards would go down since the mining rewards would have to be split up amongst more miners.
What is guaranteed is the block reward for solving a block during the time period above (it's part of the Sia whitepaper), what is not guaranteed is the number of miners and the amount of hash power competing for the rewards.
RE: SiaCoin ASIC Mining Analysis | Introduction and Overview of the Obelisk