Are these terms defined in law somewhere?
It largely depends on the context. Even the legal definition of both terms is too narrow, and thus it fails to cover important areas of [case] law.
The definition of opinion in Black's Law Dictionary refers to the judges' justification for their decision of a case. That dictionary's definition of information centers on the context of accusations ("An accusation exhibited against a person for some criminal offense, without an indictment. [...] The word is also frequently used in the law in its sense of communicated knowledge.") (emphasis added).
But the term opinion as expression of an assessment is also habitual in professions requiring duly accredited qualifications or specialized knowledge. One example is the Statements of Actuarial Opinion. It is noteworthy that, although the term includes "opinion", the actuary's assessment ought or is supposed to be premised on information that meets the standards, body of knowledge, and techniques that are requisite toward the accreditation of those qualifications.
From another viewpoint, defamation law makes a distinction between statements of fact and statements of opinion (the former might seem closer to the notion of information but, again, that depends on the context in which the term is used). Statements of fact are assertions that refer more directly to, or describe, the entity or object of which it is being spoken. By contrast, a statement of opinion refers to the speaker's materially mental/subjective impression about the entity or object of which he speaks.
The disclaimer to which you refer seems to be the author's attempt to highlight the objectivity of his article.