It seems only airgap.it is the best one for me with open-source code as an alternative for this, and the good thing is you can separate your wallet that keeps your keys offline and watch-only wallet. The only problem is it only has limited supported coins, and I hope they can add more.
An airgapped setup doesn't save your wallet's coins when weak and/or flawed entropy was used by your wallet for whatever reasons!
Weak entropy means there's a feasible attack possible for someone else to brute-force your seed because the search space is drastically lowered to such an extend that an attack is doable. This is what happened with the debacle of flawed Coldcard firmware.
If someone can brute-force your seed because your wallet generated very weak entropy due to very flawed RNG or other severe issues, then it doesn't matter if your wallet is airgapped or not.
So, I don't quite understand what you're trying to say in the beginning of the cited portion.
I would generally stay away from closed-source wallets because without reverse-engineering the binary executable you have no way to see if this wallet handles the important stuff properly and doesn't screw up the security of your keys and coins.
I think you're addressing a different security aspect from what you have quoted/ was saying.
Nobody said an air gapped wallet can protect against weak entropy. If the wallet generates a predictable seed because of a flawed RNG or implementation, then obviously keeping it offline doesn't magically make that seed secure.
The point is, about key isolation, keeping the wallet that holds the private keys offline while using a separate watch only wallet for monitoring and of course, transaction preparation. Also, that's an additional security layer against online attacks and malware, not a solution for bad entropy. From what i had keep reading from BitMaxz, he is most probably using electrum, so addressing entropy aspect is pointless since electrum is known for having good entropy.