While both Ericsson's work and the twin study feed into the nature-nurture debate, the twin study does not contradict Ericsson's assertions.
The misconception is in the following quote:
a recent twin study has stated that on average, a human's ability is 50% influenced by genetic factors, and 50% influenced by environmental factors. This seems to contradict Ericsson's experiment, as he has asserted that genetic factors influence a person's abilities by less than 1%.
The results from Polderman et al. (2015) and the assertions made by Ericsson are not this simple. Polderman et al. (2015) is a massive meta-analysis that looked at 28 general domains of performance, including things as broad as "psychiatric" and "cognitive". They analysed close to 3000 studies to do this.
Ericsson's claims on the other hand are specifically about expert ability. This means that his assertions are limited to specific domains of performance, such as expert musical ability, which the twin study does not look at. His assertions are also limited to studies that actually have experts in them. The vast majority of twin studies in the meta-analysis do not have experts involved in them.
Ericsson and Harwell (2019) discusses objections based on studies like Polderman et al. (2015) in a section of the paper titled, "Inferring Genetic Limits for the Effects of Practice on Attained Performance".
In addition, the ACE model the meta-analysis uses has limited power when talking about gene x environment interactions. This is important because Ericsson's models often incorporate the idea that genetic factors exist but are less important than practise for experts. This is less important than the points mentioned above, but still relevant when critically evaluating the paper.
Thus, the twin study only shows that for most people, if we look broadly at general abilities, they can be explained (nearly 50%) by heritable genetic factors. However, Ericsson claims that for experts, if we look at domain-specific abilities, they are best explained by practice and not genetic factors. They are talking about slightly different things and thus one does not contradict the other.