The external link you shared in that post seems to be down.
That said, as similar as they may be treated in Switzerland, one should not be too quick in lumping together 2nd and 3rd pillar and their tax treatments in other countries.
Pillar 2 is a pension fund scheme that allows for optional lump-sum payments at retirement.
Pillar 3a however doesn’t usually - and in finpension’s not at all, TMK - allow for pensions. They’re more a retirement savings scheme. And (at least) retirement savings do seem to enjoy preferential taxation rates in Spain…
I’m not a tax advisor either. But even as just an 
with a 
, if I may disagree on that…
“If the scheme is not defined as a pension scheme by Spain then the same applies but the problem is much less because retirement savings schemes enjoy a much better tax treatment in Spain.”
The tax treatment of foreign pensions in Spain
Given the examples mentioned in that article, pillar 3a with finpension would seem to qualify as such a retirement savings scheme to me. After all, you can’t get a pension, and it’s pretty similar to a self-select ISA.
And retirement savings schemes, according to that article, are taxed at capital gains tax rates …of only 19 to 26%. Which is not only no more than the marginal tax rate for someone like OP in, for example, Zurich.
Not only would she or he pay that same tax anyway, when alternatively investing the same amount into non-3a securities. Given that the tax break is upfront, the pillar 3a investment should be the better one (if the assumption regarding tax holds water and at current conditions and tax rates).