The water.
A sequestrant doesn't make Iron disappear. It just holds it so it can't react and stain things. .
Sequestrant is to Iron what the hard candy shell is to the chocolate inside an M&M. The chocolate is still there, it just can't melt in your hand. But the shells can crack. The Iron is still there but sequestrant does break down over time, hence the instructions for the maintenance dose. And the pool filter is catching it, so you're good.
In short: you're worrying about nothing.