You are correct that they did not explicitly say this, however I do believe they - through Director James Comey's statements, strongly implied this:

The San Bernardino litigation isn't about trying to set a precedent or send any kind of message... We simply want the chance, with a search warrant, to try to guess the terrorist's passcode without the phone essentially self-destructing and without it taking a decade to guess correctly. That's it. We don't want to break anyone's encryption or set a master key loose on the land.

https://www.lawfareblog.com/we-could-not-look-survivors-eye-...