This is a source-available project, not an open source project. GitHub issue #7 pointed out in January of 2021 that the project has no license, implying that the creator reserves all rights except those they must waive to comply with GitHub’s terms of service. The project still has no license today.
Also, it’s not as easy to download the project binaries as the README makes it sound. The README links to https://github.com/DanRuta/xVA-Synth/releases, but no releases have been published. The README also says to “download the app from the Nexus websites”, but that doesn’t seem possible either. For one thing, the README mentions “the nexus.com website” further down, but nexus.com is unrelated – they actually mean nexusmods.com. Even on Nexus Mods, searching for “xva synth” brings up results like “xVASynth 2 - SKVA Synth”, which sound like specific voice models, not the app itself. The one place I was able to find the app for download was on Steam as a free app xVASynth v2.