Indeed ... The modern internet originated as ARPAnet in 1969, and Sir Tim Berners Lee invented
the World Wide Web in 1989, which went public about 2 years later. So the ad should say that
the internet is one the most important inventions of the 20th Century, not the 21st.
Oh, and I should point out a popular misconception you seem to have fallen foul of in your post.
The 21st Century actually started in 2001, not 2000 as most people wrongly seem to believe.