> If it has less than 64 kB of memory how is it going to run a WASM runtime anyway?

There is WARDuino (https://github.com/TOPLLab/WARDuino and https://dl.acm.org/doi/10.1145/3357390.3361029).

A runtime that accepts Wasm modules that use a large fraction of the functionality, there is going to be a RAM requirement in the few KiB to few tens of KiB. There seems to be a branch or fork of Wasm3 for Arduino (https://github.com/wasm3/wasm3-arduino).

If you are willing to do, e.g. Wasm -> AVR AOT compilation, then the runtime can be quite small. That basically implies that compilation does not happen on device, but at deployment time.