rkyv is not like serde. There is no process of deserialization. The function rkyv::access(...) takes a &[u8], performs a consistency check, then casts the pointer it was passed to the desired type and returns Result<&T, Err>. I will say this again: "deserialization" in rkyv is a reference-to-reference conversion.
As you can imagine, it's very fast, but the tradeoffs are massive. The serialized bytes must exactly match the memory layout of a struct (usually distinct from the type being serialized) called the "archived type" (which must be #[repr(C)] or #[repr(transparent)], and which is most commonly generated from the fields of the struct being serialized using a derive macro). More importantly, there is nowhere to stick deserialization code which could convert a Unix timestamp to a jiff::Timestamp.
You can't really implement custom serialization schemes for third party types with it, at least not unless you want to define that #[repr(C)] struct yourself, and/or you're okay with actually deserializing (in rkyv, the Deserialize trait accepts a reference to the archived type and returns an owned value of the serialized type, usually by effectively cloning it) and the performance cost that entails, and with how my codebase is currently architected, I'd have to redo it every time I used the value. rkyv support kind of has to be baked into a library from the get-go for it to be worthwhile.
Pro tip: when copying a link to a Lemmy/Piefed/Kbin/etc post, please make sure you use your instance's option to copy the link to the post on its original instance (the instance the post was made on), rather than simply copying the link to the post on your instance out of your URL bar. On your instance's web interface, that option is in the More drop down next to the Boost button, just below the post title. (On a Lemmy instance, the link to the original post is the Fediverse icon right next to the upvote and downvote buttons, and you can right click and Copy Link from there).
Obviously, when you open a post in the browser on an instance that isn't yours by clicking a link (for example, this link to this same post on my instance, which is a vanilla Lemmy instance), you can't interact with it, because you aren't logged in, because you don't have an account on that instance. To interact with it, you need to open the same post through your own instance.
If you have a link to a post on the instance it was created on, you can simply paste that URL (the whole thing) into your instance's search bar to open it on there, so you can vote/comment/etc. Some mobile apps can even do this automatically. However, this doesn't work if you paste a link to a post through an instance other than the instance it was created on. In order to interact with the post you just linked to, people have to click the link to open the post on your instance, then click the button on there to copy the original link, before pasting it into their own instance. It's polite not to make them go through that extra work.
Judging by your account age, it seems you're new to Lemmy, so you probably didn't know that. Just something to keep in mind for future. :)