So far AFAIK this claim isn’t repeated by any reputable publishers. E.g. Associated Press and LA Times both published 2.5 hours after PEOPLE and did not make this claim.
Speaking of media, I found it really useless that before the names were published, the majority of news articles just said "78 and 68 year old persons found dead [RIP] at Rob Reiner's home", but I had to search for his and his wife's age to correlate that it's him and his wife. I think only 1 news article said, "authorities have not said the names, but those are the ages of Rob Reiner and his wife".
In a remarkable coincidence, the Reiners' son has just been booked on suspicion of murder:
https://www.nbclosangeles.com/investigations/director-rob-re...
Maybe the cops were reading People in between scarfing down donuts and chain-smoking Marlboros.
The claim is that there was no sign of forced entry, implying whoever did it was already in the home.
Here's another independent report: https://www.rollingstone.com/tv-movies/tv-movie-news/rob-rei...
Also, People is credible for this type of reporting. They're owned by a major company, IAC, and they don't have a history of reckless reporting or shady practices like catch-and-kill a la the National Enquirer. They likely just have sources that other news outlets don't.