This is misleading, physical things like coins or dice can have a lot non-randomness if they were not designed specifically for the purpose of generating random numbers - https://www.schneier.com/blog/archives/2009/08/non-randomness.html

If you are paranoid about your computer being backdoored to generate non-random keys, you should probably just stop using Bitcoin, since backdoored system can cause harm in many other ways - for example, generating non-random k parameter in ECDSA.

Also, generating private key inside the browser is much less secure compared to software, especially if it's done online.