Curlywhirly
MissAdventure
In my little family, having a baby was a family affair.
Not because anyone insisted on it.
Not because anyone took it upon themselves.
Just because we all loved one another and were excited.
Lucky, I know. 
Yes, same with us - our son and DDIL were so excited when their baby arrived and they were so proud that they'd given both sets of parents their first grandchild; no way could they have waited 2 weeks to show off their little one. Which just illustrates that this 2 week isolation doesn't suit everybody and I do realise not every family feel and act like mine. MissAdventure we are lucky indeed.
Same here. My DS & DiL wanted both sets of GPs to visit asap after our first GD was born ... we were all able to meet her on the day she arrived. Our second GD was born during lockdown, and whilst my DS was able to work from home, which was wonderful in that they spent so much time together after she was born, both DS and DiL were very upset that they were not able to share their joy with GPs, and missed having the help and support that we were able to offer during those early days.
Some of us are indeed very lucky that our ACs still love to spend time with us, and wouldn't want to exclude us from such important, happy, family experiences.
However, each to their own, and I appreciate that some new parents have different ideas, and that's fine. But, as others have expressed, the way in which the new parents in the OP went about it seemed rather insensitive. Not necessarily for more distant relatives, or a wider circle of friends, but particularly towards the GM. However, we don't know the full circumstances of their relationship, so it's difficult to know why they chose to do it in this way.
I'd just like to point out that the fact some new parents want to have time alone to bond with baby, before having visitors, is not new to today's generation of new parents. I had my babies in the 80s/90s, and welcomed visitors as soon as they were born (even though I had a terrible birth experience and wasn't feeling great, I wouldn't have wanted the new GPs to miss out on something so special). However, my relative who had her baby at the same time, put a notice on her front door kindly requesting no visitors ... but this was at least after both sets of GPs had been allowed to meet their new GC. This was 33 years ago, so it's not some new rule that today's generation of new parents would like to think they've invented.