I designed smoltcp (and wrote most of the code currently in it). The original TCP/IP RFC (RFC 793) contains several ambiguous requirements, and as a result they do not specify a well-defined system. There are also some outright incorrect statements. There are a few follow up RFCs (e.g. RFC 1122) that clarify these issues, and there are more RFCs (e.g. RFC 7414) that describe the TCP features that you should avoid using.

By using this collection of TCP/IP RFCs that grew over the years, it is indeed possible to implement a stack from first principles and have it interoperate with other existing stacks without much trouble. (At least so long as you don't put the same bugs in your test suite as you do in your stack... which you will.)

However, being able to transmit some bytes reliably, and having a high-performance stack that works well in real world conditions are different. You might be able to do the former from RFCs, but the latter absolutely requires a nontrivial amount of tribal knowledge that you have to collect crumb by crumb, and often quite painfully, too.

Smoltcp is somewhere halfway between. It's pretty reliable, but I am sure there is much to be improved in its operation in adverse conditions, with obscure peers, and so on.