I think, not to put words in his mouth, that he is objecting to the idea of single file libraries as inherently good or better than a multi-file library. I think the objection is more about future design choices the maintainer will make. If you want to keep it single file, it may be necessary to avoid adding some features which are too complex.

Complex features usually necessitate modularization, which is against the idea of the single file library. Modularization in c and c++ is very poor and based on having multiple files, some of which represent the interface and others that represent the implementation. I at least partly think this is his objection.