I don’t have exact numbers, but the google search that I referenced the 50% result was “what percent of first round draft picks never get a second nba contract?”, and you can imagine it’s an even higher percentage that don’t when considering draftees outside the lottery.
Yeah, there's just no way that's true. I looked at the 2018 draft. Just kind of randomly picked a year far enough out that nobody could be on their rookie deals and we'd have some kind of career earnings data to work with. Here are the career NBA earnings of all the first round picks per Spotrac:
1. Deandre Ayton - $137,159,862
2. Marvin Bagley III - $73,102,717
3. Luka Doncic - $152,231,698
4. Jaren Jackson Jr. - $110,154,078
5. Trae Young - $146,431,303
6. Mo Bamba - $39,154,236
7. Wendell Carter Jr. - $60,887,877
8. Collin Sexton - $72,052,301
9. Kevin Knox - $26,979,002
10. Mikal Bridges - $83,422,120
11. Shai Gilgeous Alexander - $117,084,613
12. Miles Bridges - $51,178,354
13. Jerome Robinson - $10,128,458
14. Michael Porter Jr. - $115,041,808
15. Troy Brown Jr. - $20,278,922
16. Zhaire Smith - $8,823,315
17. Donte DiVincenzo - $39,820,178
18. Lonnie Walker IV - $21,891,552 (note: 2nd contract was just a 1-year deal, but was for over $6 million)
19. Kevin Huerter - $58,796,679
20. Josh Okogie - $24,356,289
21. Grayson Allen - $44,051,898
22. Chandler Hutchison- $10,640,969
23. Aaron Holiday - $19,563,134
24. Anfernee Simons - $19,563,134
25. Moritz Wagner - $28,739,979
26. Landry Shamet - $30,879,455
27. Robert Williams III - $43,877,048
28. Jacob Evans - $5,467,073
29. Dzanan Musa - $5,427,165
30. Omari Spellman - $5,387,948
I bolded the players that did never got a subsequent deal of at least 2-years in length. Even of those a lot of them continued to play in the NBA and get 1-year deals for at least some period of time. Basically the last 3 picks in the first round were the only ones who never got any kind of NBA deal after their rookie deal. Those are also the only guys who made less than $10 million in the NBA. And they still made over $5 million each! If you were picked in the lottery, meanwhile, you are almost certainly still in the NBA 7 years later and are more likely than not to have made over $50 million so far in your NBA career.