The post is set in the context of letter correspondence and can be read here.
3) Throughout the history of revelation prior to the coming of Christ, Israel’s hymnody grew; new psalms were added at each significant phase of redemptive history (e.g., songs of captivity were followed by songs of deliverance, during and after the exile). It would be extremely odd, therefore, if, when redemptive history reaches its zenith, the covenant community’s hymnody would be silent for the first time ever. Of all times for singing to the Lord “a new song,” the day of resurrection is the time to do so.4) Not surprisingly, then, the songs sung by the redeemed saints, recorded in the book of Revelation, are never canonical OT psalms, and further, they are explicitly Christo-centric (not merely implicitly so). Either those songs are sinful to sing at all, or sinful to sing on earth. The first isn’t possible; the latter isn’t likely, because elsewhere in the NT the “heavenly pattern” is to be our conscious goal and pattern. We are to seek the things above (including, presumably, the heavenly praise).