How a mourner responds to greetings addressed to him by mistake

In Orach Chaim, Siman 554, it is explained that on Tisha B'Av, if someone greets you, you should respond softly and with gravity. Should a mourner also respond softly and with gravity after three days?

From the beginning of the fourth day until the end of the seventh, he responds softly and with gravity, and after the seventh, if someone greets him, he responds as usual.

Gemara Moed Katan, page 21b: "A mourner, during the first three days - is forbidden to greet, from the third to the seventh - he responds but does not greet, thereafter - he greets and responds as usual." The Meiri writes in Tractate Moed Katan, page 21b: "A mourner from the third to the seventh does not greet, but if someone greets him, he responds softly and does not need to inform them of his mourning." The Rashash questions, since from the third to the seventh he responds, what is the novelty that after the seventh he responds? And he answers that after the seventh he responds and adds as usual, as explained in Yoreh De'ah, Siman 148, Se'if 10 and Siman 242, Se'if 16, that the responder should add a little. According to the words of the Meiri, it is explained that the Meiri wrote to respond softly and with gravity only from the third to the seventh, and afterwards he responds as usual, without a soft voice and gravity.