The 13 days off (2 weeks and 3 days, you don't spend it on weekends) is just the first 3 years. For years 3-15 you get 4 weeks (20 days) per year. After that you get 26 days/year. They also get 13 days of sick leave per year with no max accrual. Regular leave maxes at 30 days accrued, you have to have 30 days or less at the end of the calendar year, anything beyond that is lost (use it or lose it). And now they get 11 federal holidays, sometimes more depending on who is president and when Christmas Eve and Christmas fall. It's not uncommon for late-career (15+ years) folks to have max accrued leave every year, which means they can potentially take 56 days + 11 holidays off the next year if they want, sums up to 13 weeks depending on how they use the leave. Of course, you can only do that about every 2-4 years. 2 years if you spend one year without using any leave, which would be pretty miserable. If you're smart about that 26 days, it becomes 6 weeks off instead of just 5 by pairing it with holidays.
They can also earn "credit hours", up to 24 hours of accrued time off by working ahead (an extra hour here and there, unpaid but becomes leave) and comp time (same, but no technical limit, becomes overtime pay if unused).
And there's now 12 weeks of paid maternity and paternity leave, which is better than most companies in the US.
Of course, the pay is lower which is why it's harder to keep technical talent. And as you move up the GS ranks it's a lot like moving up the officer ranks in the military, a shift from technical (at the lower levels) to managerial/leadership (at the higher levels). Which means there are effective caps on people who want to remain purely (or more purely) technical and avoid supervisory roles (outside of, perhaps, team or project leadership positions).