After some exploration, I figured out that Migaku uses “https://forvo.com/” to get the pronunciation of a word.
As far as I can find, the pronunciations are recorded by volunteer workers, and the sound quality is not so good. Also, since it’s not TTS, if a word is not in its base form, there might not be a pronunciation for the word.
It’s human-spoken pronunciation, so it has its own advantage, but to get pronunciation for every selected word/sentence, TTS should be used. In short, it could be used as an auxiliary material, but not the main one.
If you want to use a better AI-TTS with an add-on script, refer these posts:
AI-TTS demo: Google's new TTS model is really great - #4 by vet8t6z79pc4
Add-on having a feature substitute the LingQ original TTS with AI-TTS (used in the demo): LingQ Web Extension (Customizable Layout, GPT, AI-TTS, or so)