Based on the algorithm described in the link here: http://www.slideshare.net/kakugawa/distributed-counters-in-c... -- the insert path does not perform any reads, unless the column is already in the memtable (so no disk read). Am I missing something?

That is describing the 1072 approach aka the one in Cassandra today. Incrementing without read-before-write was one of the reasons Twitter liked it.

Unfortunately, while you can increment on a single node without read-before-write, you need to merge it with the existing value before you can replicate to the others. Read-before-replicate, if you will.

Theres a more recent, quite long discussion over https://issues.apache.org/jira/browse/CASSANDRA-4775 that boils down to, "this is actually the best we can do."