A quick note on migrating Disqus comments from Wordpress (or any other platform with integrated Disqus comments) to Drupal, to help those of who who are wondering why old comments are not being displayed on your new site, even after migrating all threads with Disqus' URL Mapper.
Initially I thought that Disqus just needs some time to process uploaded URL map, but when the comments were still not there on the next day, I had a closer look at the module source, and it turned out that it uses node/% URLs for identifying nodes to Disqus platform. Which means that my original URL map (using full path aliases for new URLs) was just incorrect.
As soon as I created a new one, with new URLs in node/% format, all old comments appeared straight away, no waiting involved.