Many YouTubers claim that some of their subscribers do not get notified when they release new videos (the "hit the bell icon" portion). They've also been claims of users being silently unsubscribed from channels. That is probably what they're referring to in your paraphrased quote.

Because it would be shocking to me if there were Youtubers who truly believed that having (or not having) new subscribers had no impact on the algorithm.

My understanding is that the percentage of viewers that are actually coming from subscriptions has gone down significantly. The recommendations drive everything, and they don't really care if you're subscribed for what videos they offer.