The plaintiffs in Trump's lawsuit filed a constitutional question in July as to the legality of Section 230. The federal court handling the case in Florida certified the question to Attorney General Merrick Garland and in late August ordered the DOJ to decide whether to intervene to defend the legality of the statute.
In its filing on Nov. 22, the government noted that the Justice Department "has an unconditional right to intervene to defend the statute" and is intervening "for the limited purpose of defending the constitutionality of Section 230."
The counsel for Facebook and Trump agreed to the DOJ's intervention, according to the filing (pdf).
Section 230 of the Communications Decency Act shields internet companies from liability for good faith removal of "objectionable" content.
On the campaign trail in 2019, Biden told The New York Times that Section 230 should be repealed.
"The idea that it's a tech company is that Section 230 should be revoked, immediately should be revoked, number one. For Zuckerberg and other platforms," Biden said in an interview with the Times' editorial board on Dec. 16, 2019.
"It should be revoked because it is not merely an internet company. It is propagating falsehoods they know to be false."
In May this year, Biden revoked a Trump order that had targeted Section 230.
In his class-action lawsuit against Facebook, Trump is seeking the reinstatement of his account, punitive damages for being banned from the platform, and for Section 230 to be declared unconstitutional.
Facebook banned Trump indefinitely from its platform on Jan. 7, 2021, the day after the riot at the Capitol.