gRPC was based on a HTTP draft that predated the standardization of HTTP/2, so presumably that statement was said about HTTP/1. HTTP/2 may not have existed at the time it was asserted.
gRPC gives you multiplexing slow request over 1 TCP connection, which reduces all the work and memory related to 1 socket per pending request; gRPC means you don't have to put the string name of a field into the wire, which makes your messages smaller which puts less stress into the memory system and the network, assuming your field values are roughly as large as your field names.
Multiplexing is a HTTP/2 feature. But as gRPC was based on an early HTTP/2 draft, it beat HTTP/2 to the punch. Thus it is likely that HTTP/2 didn't exist at the time the statement was made and therefore HTTP/1 would have been implied.
That... doesn't make any sense, since gRPC is layered on top of HTTP. There must be missing context here.