No 1 John 2:8 is not a reference to 2 Thessalonians 2:3.
A conclusion that since the “antichrists” have shown up, Paul’s objection is obviated does not necessarily follow from either Paul or John’s letters.
First, Paul prefaces the statement:
not to be so quickly upset or alarmed when someone claims that we said, either by some spirit, conversation, or letter that the Day of the Lord has already come. (2 Thessalonians 2:2 ISV)
The Day of the Lord is not going to be announced by a letter. In other words, this event will not go by unnoticed and the Thessalonians will not learn of by reading a letter.
Second, the man of sin has other qualifications:
He opposes and exalts himself above every so-called god and object of worship. As a result, he seats himself in the sanctuary of God and himself declares that he is God. (ISV)
These are different from the antichrist.
Albert Barnes notes the proper meaning of antichrist (ἀντί anti) in composition is:
(1) "over-against," as ἀντιτάσσειν antitassein;
(2) "contrary to," as ἀντιλέγειν antilegein;
(3) reciprocity, as ἀνταποδίδωμι antapodidōmi;
(4) "substitution," as ἀντιβασιλεύς antibasileus;
(5) the place of the king, or ἀνθύπατος anthupatos - "proconsul."
The word "antichrist," therefore, might denote anyone who either was or claimed to be in the place of Christ, or one who, for any cause, was in opposition to him. The word, further, would apply to one opposed to him, on whatever ground the opposition might be; whether it were open and avowed, or whether it were only in fact, as resulting from certain claims which were adverse to his, or which were inconsistent with his. A "vice-functionary," or an "opposing functionary," would be the idea which the word would naturally suggest. If the word stood alone, and there were nothing said further to explain its meaning, we should think, when the word "antichrist" was used, either of one who claimed to be the Christ, and who thus was a rival; or of one who stood in opposition to him on some other ground. [Barnes' Notes on the Bible]
The antichrist is anyone who attempts to take the place of Christ. The man of sin is a singular entity that attempts to take the place of God by seating themselves in the sanctuary of God and delcaring that he is God.
The simplest explanation is that the antichrist and the man of sin are two separate and distinct entities. Revelation describes the counterfiet system of those opposed to God and it includes the dragon, the beast who gives authority to the dragon, and the two false witnesses. There is one leader and yet they do not act alone.