I'm not sure if you are asking about security with respect to IPv6 specifically, or more generally, security in IoT. I don't think you can make the case that IPv6 has some inherent security benefits, although in some small ways, this may be partially the case.

In the earlier days of IPv6, until not too many years ago, the thinking was that IPv6 would always be deployed along with the layer 3 IP Security protocols (IPsec, see RFC 4301 and other related RFCs). But many people opposed this mandate, because many organizations were using equally good security protocols, at other protocol layers, already, with IPv4. Besides, IPsec is just as applicable to IPv4 as it is to IPv6. So the IPsec mandate no longer applies to IPv6, and there is no reason to think that IPv6 has any significant security benefits over IPv4.

Well, to be more exact, the much greater address space does offer some benefits. As of now, most IPv6 subnets use 64-bit Interface IDs (IIDs). Makes it much more difficult to attack individual hosts, if you have 64-bits to run through, than if you only have 8 bits or fewer, as in IPv4.

So the security issues with IoT are real, but not so much unique to IPv6. There are security protocols available at any of the protocol layers, which can be adopted in IoT environments. Certainly MACsec (layer 2), IPsec (layer 3), and Transport Layer Security (TLS, at layer 4), would be candidates. TLS is used quite a bit on the Internet, because it only involves the applications at both ends. I would expect that TLS would also be a good candidate for IoT. And also less involved security mechanisms, such as providing security gateways between IoT devices and the Internet, air gaps, or what have you.