Timing is quite important in the longterm. This article is saying that you can still make a profit despite bad timing (assuming the stock market does well), not that timing is not important.
It also only analyzes a single time period (1970s - 2015), which happens to be quite a good one for stocks, and then makes an implication about markets in general which is quite speculative.
Yes, but the point is: even with the worst possible timing you still get ahead of inflation ($1.1M from $184k invested). With regular investments you could double the profits. Also, 1970-2015 period includes three big crises, with crashes of around 50% (1972, 2000, 2007), and a bit milder crash of 1987 (34%), calling it "quite a good one for stocks" is kind of a stretch.
Notably ~1917 is when global power shifted from the U.K. to the U.S., cemented in 1945.
(not sure if they included dividends - would probably change the calculations, but for sure you see that 1815-1900 was a ~85 year general bull market in U.K. stocks, followed by ~75 years of real-term declines from 1900-1975).
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Also, the U.K. stock market went nowhere from 1874 to 1952 in pound terms, a whopping ~70 years!
