In this case, the author refers specifically to the UK QROP scheme, but this is similar to an IRA in the US or and RRSP in Canada.
The gist of the article is this: Spain doesn’t recognize these accounts as anything other than a foreign investment account. You don’t get to deduct any contributions and when you withdraw your money, you’d be expected to pay taxes on any gains made in the account.
QROPS are more restricted in terms of withdrawal date, so the income can grow tax free until you withdraw it. This does not appear to apply to an IRA or RRSP (since you can withdraw these at any time), so you’d have to pay Spanish taxes on any dividends, interest or capital gains received from account, just as you would for any other investment account.
Of course, you would still owe US or Canadian taxes on your withdrawals.
For US citizens, if you’ve been working in Spain and pay higher taxes here than you would in the US, you probably have some excess foreign tax credits you could apply to reduce the amount of money you’d pay to the IRS.
For Canadians, you are stuck with paying the 25% withholding tax on your RRSP when you withdraw it (still better than what you’d pay in Canada!). I haven’t looked into how foreign tax credits work here in Spain, but I’d imagine that if you have non-Spanish income, you could use the 25% you paid as a tax credit against Spanish taxes you would owe on your world income.