Not in my experience on large websites. If "all it takes" is round tripping the audio component through speech-to-text, then it's apparently annoying enough at scale to stop the vast majority of spam.
Also, automating something in a browser extension is basically the easiest place to automate something because it's just done once per user, with their cookies/sessions/ipaddr, very infrequently.
So that a browser extension exists for something doesn't mean it's trivial at larger scale. I doubt Google cares about the usage pattern that Buster gives people since it alone is not abuse, just normal people filling out the someodd <form>.