It
wasn't too difficult to achieve, but past performance is no guarantee of future performance.
Already that's looking far less true if you look at the past decade or two, but it's still the received wisdom because it's what people enjoyed in the past.
The FTSE 100 hasn't peaked much higher than it peaked in 99/00, and in real terms still hasn't closed as high as it did back then.
Since you were careful to talk about nomimal return, this is then further reduced by the effects of inflation. A 5% nominal return might well be easy in an environment with a high inflation rate.
I'm not suggesting that people shouldn't invest, people clearly should invest as part of a mixed portfolio, and because cash returns are typically even slower, but the original article here is unrealistic as a goal.
Edit: Dividends help too of course, they aren't tracked into the index whereas it is possible to reinvest them back in if you don't take them as income.
I'm not denying that some people can get lucky with market timings and it works out well for them, and in fact there's the whole boomer generation it mostly worked out very well for, but it does no good to the next generation to account it all towards their actions and not account for any kind of good fortune in timing.