> Is that really true?

For Python by Guido (this was still in the BDfL era)? Yes. For scripting languages generally? Also yes. For computing as a whole? While parallelism was more important than in either of the preceding contexts, it was still far less important than today, so, again, yes.

> We already had multicore machines, and Herb Sutter's "The free lunch is over" article had been published for years by then.

Barely, unless you are talking about multi-CPU SMP machines (which existed for PCs back to the 386 era); the first multicore x86 processors were released in May 2005 (And Sutter’s article was in March) about a year before the major decisions about Python 3.0 (published April 2006.)

> Herb Sutter's "The free lunch is over" article had been published for years by then.

Python 3.0 was released in 2008 with the work starting in early 2006 (maybe earlier, going by PEP 3000 which was published in April 2006). Herb Sutter's "The Free Lunch is Over" was first published in 2005. I don't think a year between its publishing and the work on Python 3 beginning qualifies as "years".

Python is used much more widely and for more data tasks now than it was then, I think. Besides, we've all slowly adapted to using parallelism everywhere, it was not overnight.

I'm sure parallelism was understood and worked towards in computing at large and within certain programming languages. The then contemporary and popular use-cases for Python (which were they?) might have had very different challenges solved by other means.

(I'm just guessing here)