"He pointed out in his article that many journalists have done many mistakes when they wrote the text"
Со временами, похоже, путаница. . временами случается. . ИЛИ "He points out"...
В соответствии с доктриной "to make money, mistakes... for oneself" (a "to do - for the rest") можно предположить, ЧТО to make mistakes = ошибатьСЯ, заблуждатьСЯ, в то время КАК
to do mistakes = допускать ошибки намеренно. .
КАК ВАРИАНТ: "He pointed out in his article that many journalists had done many (a lot of) mistakes (while) writing their text(s)"