In a Friday blog post, Armstrong also addressed questions about whether Yahoo would cease to exist as an independent ad platform and why advertisers would continue to advertise with Yahoo after the deal.
Yahoo plans to continue to serve its own ads, he said, and Yahoo plans to use Google ads primarily on Web pages where few or no ads now appear. "The only way for an advertiser to guarantee placement for their ads on Yahoo is to advertise through the Yahoo platform itself," he wrote.
The Google explanations didn't sway some critics.
Google has attempted "to weave and spin this critical issue," said Jeffrey Chester, a frequent Google critic and executive director of the Center for Digital Democracy, a privacy and open Web advocacy group.
The issue isn't as simple as Armstrong's posts suggest, and Yahoo's decision to turn over a core part of its search functionality could leave the company "fatally wounded," he said. The deal also raises privacy concerns, Chester added.
"Yahoo's future, in my opinion, as a full-service online ad company is endangered, as more businesses realize that its search ad business relies increasingly on Google," he added. "Google should have been able to acknowledge that a major deal with its leading search competitor raises serious questions worthy of broad debate and critical analysis."