I assume you're talking about J Philippe Rushton.  His research didn't focus on twin studies as much as Nancy Segal.  Here is an interview Stefan Molyneux had with her.  TLDR they replicate and strongly show IQ heritability.

https://odysee.com/@freedomain:b/nature-or-nurture-the-study-of-twins:d

The environment does have an effect, but it is limited by your genes in the same way that improved nutrition doesn't enable infinite height growth across generations and different genetics results in significant height difference within and across populations.

The Flynn effect that was seen in western countries was seen to be mostly on non g domains and FSIQ tests didn't see extreme increases in IQ.  In Europe the Flynn effect began reversing in the 1990's and in the US it was seen in the 2000's.