This is not true under 18 U.S.C. § 1503. Read this very carefully: "Whoever, with intent to avoid, evade, prevent, or obstruct compliance, in whole or in part, with any civil investigative demand duly and properly made under the Antitrust Civil Process Act, willfully withholds, misrepresents, removes from any place, conceals, covers up, destroys, mutilates, alters, or by other means falsifies any documentary material, answers to written interrogatories, or oral testimony, which is the subject of such demand; or attempts to do so or solicits another to do so; or
Whoever corruptly, or by threats or force, or by any threatening letter or communication influences, obstructs, or impedes or endeavors to influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States, or the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress—
It is hard to prove intent... but, not impossible. So far the President has not impeded or had any influence in any legal process. Second, one can state their opinion; an elected official does not give up their Constitutional rights when assuming the office. By saying that he, as the President, could pardon any of the people being persecuted (and yes, that is the term I intended) by Mueller is not obstruction. It is stating a fact, he could pardon them. It is not a threat or an stumbling block for Mueller, it is simply a fact of the Constitution that gives him that power.
When he puts it out to the press it's called obstruction which is a crime he doesn't have to do it saying it is enough!!