If a judge finds they've established themselves as a trustee, this argument won't fly. In fact it will make a quite spectacular and expensive thudding sound.
The trustee-beneficiary relationship arises from the situation and does not require a contract to be formed. If you are the legal owner of assets "for the benefit of" someone else, congratulations, you are probably a trustee.
Why does this matter?
Because trusteeship comes with a fiduciary duty. Fiduciary duty is a heavy burden. If it's applied to Brave it will create merry havoc: a pile of money that they cannot touch, under any circumstances. A pile of money that they must return, if it cannot be forwarded to the intended recipient. A pile of money that cannot be mixed with anything else in any way. The requirement to put the interests of the beneficiary ahead of their own. And on and on.
It's an attempt to be clever at marketing, to create an incentive to sign up. But as a legal situation it's a swamp full of unstable turd grenades.
... of course, I am not a lawyer and this is not legal advice. Maybe Brave found a friendly jurisdiction or a quirk in trusts law that they can squeeze through. But given the history of startups wishing that law doesn't real, I kinda doubt it.