Specifically so that wallets can't say exaggerations like this, a wallet security commitee needs to be formed.
Its members should include contributors to various open-source wallets, as well as security professionals working for the big wallet companies.
Their sole function would be to review the source code of every wallet (an audit) and then assign it a rating like A+, A, etc. It would also give out 0 ratings to wallets which aren't code-signed (not a problem as you can buy these from second-hand TLS sites for $60/year). In my opinion, all wallets should be code-signed by a reputable CA (even Electrum, eventually).
The rating would be the only benchmark you are allowed to advertise in your wallet.
It worked with UL Benchmarks I don't see why it wouldnt work wih code & software.
The issue is with the funding. You cannot possibly get enough funding to fund such an organization. The security professionals, or really any developers don't have that much time or money to audit codes all the time. The current system as it stands doesn't really have much problem; you have contributors auditing and several with commit access to push the changes. Wallets are generally not advertised because they rely on donations, except those that run some sort of services. If all the wallets were to come under the purview of some organization, then you would find tons of bureaucratic red-tape surrounding it. I'm sure most would rather not have this sort of stuff.
Code-signing doesn't do anything but provide a false sense of security. There has been instances where certs were stolen and used to sign fake versions of certain wallets (Electrum) for example. Making them untrustworthy based on this alone sounds quite unfair. Anyways, isn't Electrum code-signed?