my background is mathematicsYou are a terrible example because you come from a formal background of a hard science. Even worse, you come from one where proofs (the cathedral in the mathematics sense) are required or you aren't taken seriously.
The parent was talking about joe schmo off the street or the high school wunderkind who is building "twitter for teens" or "pinterest for social good". They are just slapping it together from the get go without thinking about any aspect of the design. Eventually they'll have to hire the formally schooled to come in and clean it all up, if they actually get anywhere with it.
Which makes me wonder, if more formally educated individuals were in the start up game would the failure rate be so high as it is now?