That video shows the Mocha Pro Adobe Plugin interface in After Effects, not the light Mocha AE plugin.
The matte clip dropdown hasn’t been in Mocha AE for several years.
The much older standalone version of Mocha AE had external clip support when it was separate from the AE application, but it was removed when the plugin version was added to After Effects. The transition to 64 bit After Effects meant we had to drop the 32-bit external Quicktime import support process when we bundled the plugin.
We retained the insert clip dropdown for preview purposes, as those clips are built into the application.