Actually, it's you who is giving that impression with an ultra vague "doesn't solve the problems described".

The only problem in the blog post is efficient coding of optional fields and all they was introduce a bitmap. From that perspective, JSON and XML solve the optional fields problem to perfection, since an absent field costs exactly nothing.

I guess you missed the part where the size of the data stored on disk and efficient deserialization are also critically important performance characteristics that neither JSON nor XML have?

Capnproto doesn’t support transform on serialize - the optional fields still take up disk space unless you use the packed representation which has some performance drawbacks. Also the generated capnproto rust code is quite heavy on compile times which is probably some consideration that’s important for compiling queries.