Are you wondering if such technology exist, or are you wondering if the speech of a company you know but for some reason do not mention is realistic from a technical point of view?
No matter what, such company exist, an example would be Amourcard. I do not know if there are other, they claim to be the first to offer such service.
This company targets more specially the NFC world, which is subset of RFID used mainly for credit card and passport:
Armourcard electronically jams the frequency these credit cards &
epassports communicate over (13.56MHz)
NFC technology does not cope with noisy environment. I mean that if the communication frequency is full of garbage information, the communication between the NFC reader and the tag will not be possible.
This card goal is therefore to produce such garbage information. Internally it is most probably exactly the same kind of circuit you may find in NFC enabled credit card, but instead of implementing the NFC communication standard it just endlessly send garbage as soon as an NFC reader tries to communicate (by the way, on their OOperating Instruction page, section 'Tap & Go' payment gateways, they acknowledge that this behavior may lead several readers to crash and reboot).
- At least in principle, this should effectively work for the designed purpose, which is protecting NFC enabled credit cards and passports, as long as this jammer card is stored together with them,
- It may be less efficient with other kind of RFID tags, even if they operate on the same frequency, due to protocol or technical differences,
- It will not be effective at all with RFID tags operating on different frequencies than 13.56MHz.
Quick addendum: while browsing the site of the aforementioned company, I see that they made the choice for their jammer card to embed its own battery instead of relying on the NFC reader signal like classical NFC tags. They announce a battery life within 18-24 month. Be sure to check the battery status regularly (a green led lightning up when you temporarily disable the jamming feature by holding your finger on a small "button" on the card), since when the battery deplates you will not be protected anymore maybe without noticing it (the card needs to be replaced by new one, no way to replace it).