[FOM] The Irrelevance of definite descriptions in the Slingshot Argument?

[A.S.Virdi@lse.ac.uk]

Dear FOMers,

Can anyone think of any significant mathematical difference between the
following two arguments? 

1. s                                                    Premise
2. {x: x = d & s} = {x: x = d}  From 1., given substitution salva
veritate of logical equivalents
3. {x: x = d & t} = {x: x = d}  From 2., given substitution salva
veritate of co-referring terms
4. t                            From 3., given substitution salva
veritate of logical equivalents

And (with i is the iota/definite-description operator)

1. s                                                    Premise
2. ix(x = d & s) = ix(x = d)    From 1., given substitution salva
veritate of logical equivalents 
3. ix(x = d & t) = ix(x = d)    From 2., given substitution salva
veritate of co-referring terms
4. t                            From 3., given substitution salva
veritate of logical equivalents

Both arguments seem valid (don't they?). So why has there been much
philosophical ado about nothing concerning the status of definite
descriptions in setting up this slingshot argument? Replace definite
descriptions with their set abstract counterparts and there are no
iota-expressions to be concerned with. Am I missing something here?

Arhat Virdi