In the tech industry, the average annual turnover rate is 13.5%. If I'm doing my math right, that amounts to a lifetime turnover of 77% for an average 10 year tech company assuming flat employee count over the time period, or 51% for the same company with linear growth from zero over the time period. Both are well above the 10% this company is claiming.
(The 77% came from 1 - ((1-0.135)*10), and the 51% came from 1.0 - ((1-0.135)*10)0.1 - ((1-0.135)*9)0.1 - ((1-0.135)*8)*0.1 ... to sample even cohorts of 10, 9, 8, etc. years making up equal portions of the population.)