The author of the arstechnica piece (who, unlike the author of the Washington Post piece, is not a laywer) hasn't really explained things well. It's worth reading the actual decision from the court[1].
A relavent paragraph:
In Fisher, however, the Court also articulated the “foregone conclusion” rule, which acts as an exception to the otherwise applicable act-of-production doctrine. Fisher, 425 U.S. at 411. Under this rule, the Fifth Amendment does not
protect an act of production when any potentially testimonial component of the act of production—such as the existence, custody, and authenticity of evidence—is a “foregone conclusion” that “adds little or nothing to the sum total of the Government’s information.” Id. For the rule to apply, the Government must be able to “describe with reasonable particularity” the documents or evidence it seeks to compel. Hubbell, 530 U.S. at 30.
Note that the issue with this question is the "existence, custody, and authenticity" of the evidence not its actual contents.
A relavent paragraph:
In Fisher, however, the Court also articulated the “foregone conclusion” rule, which acts as an exception to the otherwise applicable act-of-production doctrine. Fisher, 425 U.S. at 411. Under this rule, the Fifth Amendment does not protect an act of production when any potentially testimonial component of the act of production—such as the existence, custody, and authenticity of evidence—is a “foregone conclusion” that “adds little or nothing to the sum total of the Government’s information.” Id. For the rule to apply, the Government must be able to “describe with reasonable particularity” the documents or evidence it seeks to compel. Hubbell, 530 U.S. at 30.
Note that the issue with this question is the "existence, custody, and authenticity" of the evidence not its actual contents.
1. https://arstechnica.com/wp-content/uploads/2017/03/rawlsopin...