Well, if the huge purchase (like my Venza, for example, which was a huge purchase from my perspective) is something that they have been saving for for 10 years before they retired, and the money for it was set aside apart from the portfolio, then maybe that is how they manage to do it? I dunno. That is how I bought my Venza, anyway.
I would suggest just computing the SWR based on your portfolio at the time of retirement MINUS the big thing, or things, that you planned to buy in the first few years. As I recall, you don't spend all of your 4% so after a few years of retirement, you would have set aside enough from your SWR money to buy more big things.