P. Tiersma, Language of Defamation (66 Tex. L. Rev. 303) succinctly characterizes defamation as an unprivileged accusation (regrettably, the article is not available for free in the wild) – the article goes into detail about the nature of speech acts and the difference between accusing and reporting. When one files a lawsuit against another for defamation, one is accusing another, but in a privileged context. But when one publicly declares the same accusation, there is no privilege. It is not necessary that Alice repeatedly make an accusation, and the accusation can be made to just one other person. When Alice legally alleges defamation (communicating to her attorney, or in her complaint) she has privilege, but can communicate that accusation in a very limited way. The act of filing a defamation lawsuit does not render her Twitter accusation non-defamatory.
Incidentally, it is typically a sufficient defense to a charge of defamation if the accusation is shown to be true, but this is not necessarily the case. In the old days (in England), truth did not make a statement non-defamatory, and John Peter Zenger attempted but failed to establish that precedent. One may however argue that "defamation" is a modern construct only somewhat related to "seditious libel", which Zenger was charged with.
To this day, Art. 353 of the Philippine Penal Code says
A libel is public and malicious imputation of a crime, or of a vice or
defect, real or imaginary, or any act, omission, condition, status, or
circumstance tending to cause the dishonor, discredit, or contempt of
a natural or juridical person, or to blacken the memory of one who is
and Art. 354 says
Every defamatory imputation is presumed to be malicious, even if it be
true, if no good intention and justifiable motive for making it is
the point being that a true statement can still be defamatory, in the Philippines (and to some extent, in Indonesia, discussed here).