Is that true even for mp3 though? Apparently it actually has a universal browser support up to MSIE 9 (!), while Ogg Vorbis and FLAC are now supported by all modern browsers after MSIE [1]. Opus seems to be the earliest format with less support than that, while this library only dates back to 2020.
Sure, but what if you don't want to use mp3... codecs aren't created equal. For anything latency sensitive mp3 is a non starter. For anything under bandwidth constraints, flac isn't great.
OPUS is a descendant of CELT (which never caught on, for reasons) which is an open/unhindered by patents low latency lossy codec suitable for VoIP and live streaming. It's relatively recent, but also, those use cases are more popular recently in the browser.
And to go even more extreme, some applications really care about bit-exactness in their decoders, which you don't get by relying on other vendors.
I mean, WebAssembly Opus decoder seems meaningful even to this day, but I wondered why earlier formats are there in the first place. The bit-exactness is a good argument but as you've said that's a very specific requirement, and the library only seems to care about performance and minified size anyway.
[1] https://caniuse.com/?search=audio%20format