Packer, yes, you are confused as the wording you are reading.
What IB is saying could be rewritten as this:
***You can't borrow foreign currencies.***
If you have $100k USD in an IRA, and you want to buy XYZ on the Spanish exchange, XYZ trades in Euro, so you can't buy it without Euro. So at IB (in an IRA) it's two steps, you "buy" Euro vs. USD, and then you buy XYZ (with the Euros).
In a taxable account, IB allows margin, so they can combine the forex / XYZ purchase into an attached order (they can, it's not the default) where the buy / borrow happen simultaneously. I think the reason they can't attach the forex order in an IRA is that there is no guarantee on pricing since forex and equity moves could theoretically gap between purchases... because IB is not acting as a forex dealer (like Fidelity is... charging 1%), they are just executing forex at the best bid/ask on the OTC market, they can't "guarantee" that a simultaneous dual trade like this would be limited to a specific value. (This is probably why Fido just says "they can't do this in an IRA" which is total BS, it's just that they can't guarantee you won't use margin if you executed badly in a volatile market... I wonder if Fido let's you trade overseas if your taxable account isn't a margin account? Anybody know?)
Does that make sense?
Thanks,