SQL Server 2017 CU3 will not add read-scale replica. Thinks there is a cluster
I finally found Creating a clusterless availability group using windows server 2016 and sql server vNext CTP 1.4 on Stack Overflow, which got me on the correct path.

I had been using the wizard to create the AG, when that failed I used the wizard to script out the t-sql and run step by step. What I failed to notice is that the wizard creates the replica on the primary, but never actually joins the replica on the secondary.

That is, this command executes on the primary:

ALTER AVAILABILITY GROUP [XXXXX]
ADD REPLICA ON N'YYYYY' WITH (other stuff)


but this command never runs on the new secondary:

ALTER AVAILABILITY GROUP [XXXXX] JOIN WITH (CLUSTER_TYPE = NONE);


And apparently the cluster_type = none is critical. At any rate, I am watching my clusterless AG successfully build now, so woohoo! And maybe worth mentioning that I am using SSMS 17.5. The wizard code seems to need an update to include the missing step. 

As a follow up. I worked with MS through my company's support agreement. They verified there were a few scenarios that exhibited the wizard behavior described above. They are targeting SSMS 17.6 to incorporate the updates.