This is illegal in the US too (defamation per se). The main difference is that the US requires a higher standard for public figures (proving actual malice i.e. that they lied knowingly and maliciously).

The Constitution does not apply between interactions between private citizens. The Constitution only applies between the State and the Public.

Technically Canada (and most other Liberal democracies) have similar freedom of expression (which includes speech). Where the difference lies between Canada and the US is in the Canadian Charter of Rights' structure vs. the US Constitution's structure.

In the US, "Freedom of Speech" is the first amendment, and as such (as I understand it) stands largely on it's own as an enumerated right. Unless it intersects with another Constitutional provision, or with the interpretation of the text of the first amendment itself, it's otherwise unlimited.

In Canada, Freedom of Expression is provided for in Section 2 of the Charter, but Section 1 provides for the limitation of any of the following rights and freedoms:

The Canadian Charter of Rights and Freedoms guarantees the rights and freedoms set out in it subject only to such reasonable limits prescribed by law as can be demonstrably justified in a free and democratic society.

I Am Not A Lawyer, but the legal framework for testing Section 1 laws is call the Oakes test, and the language in this article bears the hallmark of justifications of the application of Section 1:

In its decision, the court found Webster’s statements failed a basic “public interest” test since “perpetuating hurtful myths and stereotypes about vulnerable members in our society” does not represent speech anti-SLAPP rules are “intended to protect."

I would assume that since this was an attempt to dismiss a lawsuit using Ontario's anti-SLAPP law, that the motion to dismiss was overruled because the anti-SLAPP laws were in line with promoting a Free and Democratic society.

Also there is Section 33, the "notwithstanding clause", which allows for the temporary suspension on just about any of the rights by the legislature, but that's not relevant here, ans is fairly rarely used (except in Quebec).