The assumptions in that math are wrong anyway. Once you depend on 10 people, the chance that they each achieve "95% successful execution" is 0.
This is only partially down to the impossibility of having every staff member on a project be A++ players.
There is coordination RISK not just coordination overhead. Think planning a 2 week trip with your spouse with multiple planes/trains/hotels, museum/exhibit ticket bookings, meal reservations, etc. Inevitably something gets misunderstood/miscommunicated between the two of you and therefore mis-implemented.
Now add more communication nodes to the graph and watch the error rate explode.
That's what the math is reflecting. Project succeeds if all of 10 people does their job well. Each person has a 95% chance of succeeding. 0.95^10 ~= 60%, and so the chance that all 10 people do their job successfully is ~60%.
Those jobs also include things like management and product design, and so the coordination risk is reflected in the 5% chance that the manager drops the ball on communication. (As a manager, I suspect that chance is significantly more than 5% and that's why overall success rates are even lower.)
That's what I mean "only 5%" encapsulating all failure modes (comms/implementation/coordination/etc) is very low.
And that under-estimation compounds to make the top level 60% much higher than it should be.
A 7.5% rate takes top-level success odds below 50% - 46%. A not unrealistic 10% takes the top level down to 35%.
Etc.