Didn't those happen nearly at the same time? Postgres95 (which PostgreSQL started from) had support for some subset of SQL (including inner joins): https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f... outer joins were added a bit later: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit...
So why didn't people use SQL in Postgres95? The licensing was fine. My best guess is that the SQL support wasn't yet good enough to be useful. The database as such (transactions, data types, persistence) was already pretty solid, as I understand it.
Or other things have grabbed your interest. Are you saying that one lacks "intellectual curiosity" because they have had other priorities than learning this one specific technology?
If we were talking about cuckoo hashing, 386 assembly, or FIR filter design, I would agree that it's "one specific technology" that someone could easily pass over with little loss in many programming fields. But not knowing how to do a join is more like not knowing how to open a file or use floating-point math: it's a crippling deficiency in your skills as a programmer, one that will slow you down in a wide variety of tasks.
This was about the time MySQL came out, and years before Postgres added SQL support, so at the time your rather pathetic argument might have had merit, because there wasn't actually a way to get hands-on experience with SQL without a pricey license for proprietary software.
I still don't think it's reasonable to claim that you have "SQL experience" if you haven't touched joins. That's like saying you have JavaScript experience but don't know how to define a function.
Now, though, Skype and every browser embed SQLite. If you don't know enough SQL to do a join, it's because you lack intellectual curiosity. Don't blame your employer.
When I was hiring people, I wanted people who could do the job, not people who would lie that they could, then blame my company.