I think you got an older version of that interface and it should say view instead of constant. If I remember correctly, constant was the predecessor to view.
For more details on the ERC20 token standard refer to the EIP20 GitHub page:
They also link to example implementations. This probably helps ...
Founder of Sablier here. You have not been scammed, the "immediate transfer" you're talking about is just how our protocol works (we transfer the money from one contract to another one).
GSWAP is using Sablier to stream the LP tokens to you - think of this like vesting shares but in DeFi fashion. We have a FAQ that explains what money streaming is.