What's wrong here? As long as you are investing money that you can afford to lose, the there is nothing wrong with buying. Keep buying! You won't regret on the long run. Just don't sell when the market is crashing.
The problem is that people will panic when the market starts crashing and probably they will be selling at a loss. If you have a long plan then it is good to wait for the market to go down and then invest for the long term rather than investing when the market is rallying and if you are planning to make a decent profit it is the best solution and the risk is lower, in short just be smart when to invest.
I think the reason why the person you are quoting above does not see anything wrong is because this is already a great entry point, I have always thought that buying during an all time high is simply a bad idea since you are risking the price goes against you and then you face huge losses, but right now the correction already happened and I doubt we are going to see another correction of the same magnitude so this is the perfect moment to enter the market for a good price.