It's that you want to blanket ignore people that makes it feel unwelcoming. When you individually mute or ignore someone you have at least seen them around and dislike them as an induvidual and not a group.
But the counterpoint to that is the temporariness, to coin a word, of the measure. KB isn't asking for a permaban of all new users. You're comparing individual mute, a permanent action, to mass-temporary mute, a non-permanent action (as KB suggests it). It's apples and oranges in a way. The temporariness of the action mitigates its potential "unwelcoming-ness" as opposed to straight ignore, which is permanent and generally requires a lot more action on the part of the ignored to reverse.
That is a good point and I'm not against it. I could be helpful in making sure that newcomers got a friendly welcome since only those who want to read new users posts read them as well as making sure kb and others don't have to deal with new users which I'm sure is exhausting to do over and over again. I fear that it might become the norm ignore new users though. I'm sure not everyone or even a majority would do so but it is a potential downside. The action of making sure that you don't have to interact with new users is unwelcoming however you frame it though. It sends the message that new users are not welcome and their input not valued. That is not to say that it is automatically a bad idea, but that message is going to be there no matter how nicely you say it. There are downsides and upsides to the idea and I'm personally conflicted about it all.