The phrase "asserted plainly" is correct and usable in written English.
It can be used when you want to indicate that something has been stated clearly and without ambiguity.
Example: "The scientist asserted plainly that the results of the experiment were conclusive and could not be disputed."
Alternatives: "stated clearly" or "declared openly".
Writing like that asserted very plainly that the world could be conquered.
"The president of the United States and the secretary of defense would not assert as plainly and bluntly as they have that Iraq has weapons of mass destruction if it was not true, and if they did not have a solid basis for saying it," he said.
And in Treatise 1.3.10, "Of the influence of belief," he seems to assert very plainly that some causal beliefs do cause motivating passions, specifically beliefs about pleasure and pain in prospect.
Only if Bush plainly asserts our national interests will his first foray to Europe be judged a success.
What little is left in law of the Magna Carta plainly asserts the fundamental importance of presumption of innocence, trial by jury and habeas corpus.
That rosy belief might be undercut when even Larry Summers, as big an architect of the system as they come, plainly asserts that Piketty's grim predictions of the future fail to take in the "devastating consequences of robots, 3D printing, artificial intelligence, and the like for those who perform routine tasks".
In a brief filed on November 13th in the D.C. Circuit, the government asserted that the new law "plainly affords an adequate and effective substitute for any applicable habeas right".
"The city's regulation of highway billboards has more holes than Swiss cheese, demonstrating plainly that its asserted interest is nothing but a pretext for the city's true purpose for its regulation: to eliminate competition and make money for itself," lawyers for some of the companies wrote.
"Plainly, the government wants only foxes guarding the henhouse," he asserted in his motion.
It is reasonable to be wary, these days, of powerful figures asserting stories that seem plainly contrary to observable events.
Keith Phipps of The A.V. Club gave the film a 'B+' grade, and asserted that it was "a reconsideration of what constitutes Star Trek, one that deemphasizes heady concepts and plainly stated humanist virtues in favor of breathless action punctuated by bursts of emotion.
