What was the holdup? I've asked Microsoft for comment, but no word back yet. The February 22 blog post announcing availability noted that since public preview, Microsoft has been working with customer to "validate, finetune and refine the service." Update: A company spokesperson said that statement in the blog is all that Microsoft will share on the timing.
Windows Defender ATP is built into Windows 10 Enterprise. Defender ATP is different from Windows Defender, Microsoft's free antivirus service that's bundled into various Windows versions. Defender ATP (codenamed "Seville" is a post-breach service, meant to help detect threats that have made it past other defenses, give users means to investigate breaches and offer suggested responses.
Starting with the Windows 10 Fall Creators Update, Microsoft officials began describing Windows Defender ATP as a suite of tools, which includes Defender Application Guard, Defender Device Guard, and Defender Antivirus.