It's unfortunate they didn't include any references for this claim, which seems pretty bold in the face of voluminous evidence that income inequality has increased dramatically in those same "recent decades". (See e.g., https://www.pewresearch.org/social-trends/2020/01/09/trends-..., https://ourworldindata.org/income-inequality, https://www.cbpp.org/research/poverty-and-inequality/a-guide..., etc.)
The only way I can see to conclude that labor's share of value created has remained anywhere near stable is to either lump executives in with labor, or exclude passive income from the definition of value created.