Despite the very public interest of Barcelona, Nico committed to a new 10-year contract with Athletic on Friday through to 2035. The new deal also sees his buyout clause rise from €58m to €100m.
De Marcos, who retired at the end of last season, told Radio Euskadi: "He was very prudent; many in his place would have gone with the flow and left, either to Barça or Bayern.
"But he decided to stay. Not only that, he's extended his contract for several seasons. Nico remains at Athletic Club, and that makes us even more unique.
"What has happened is of incalculable value and Athletic should be delighted. You see the length of the contracts of the backbone of the team and you can only be excited.
“Nico has belonged and continues to belong to Athletic. At no time has he stopped doing so. He has never made any statements; he has been very cautious."