I like your language. Of course, GS, what do you expect.
Back to loans, yes, all banks' loan T&C will have catchall terms to protect them. The contract is bullet proof.
For example Citi's T&C in your link, no need to look far, read the very 1st line, it says:
1 Interest
1.1 We may vary the interest rate(s) from time to time at our discretion after giving reasonable notice to you.
First line already cover Citi totally for any interest rate revisions. what sibor, what spread, what fixed rate.
I feel bad for my friend. I was the one who recommended that package to him. As far as I know, all these years, no other bank has ever invoked that term to change a promised spread/rate just like that. Well done Citibank !