It's probably a mixture of some being appropriately rolled, and most at these stakes not. Sometimes pros may be taking a controlled shot and playing over what their *normal* BRM plan dictates, but yet have enough left to still be able to grind and make a living at their usual stakes. Grinding 40,000+nl poker everyday is probably not an everyday occurrence for ANYONE -- otherwise those nosebleed stakes at Full Tilt wouldn't sit empty for more than half the day.
I also think that some "pros" are more or less gamblers, and do not really follow appropriate BRM or will go on tilt and throw it all out the window. But aside from these, sensible pros can take nosebleed stakes shots and still be appropriately rolled to make a living at lower stakes.
With respect to pros taking controlled shots: let's say you are a pro that usually plays 12 tables of 600nl at a time and makes a comfortable six-figure annual salary off of it. According to your 40 buy in rule, said player would need a bankroll of at least $24,000 to sustain this. Probably more likely a roll of $50,000-60,000 at the minimum for a true pro to do this and rely on it to support him or herself.
The above 600nl pro would not need to really feed his 600nl bankroll much more beyond $60k, and he's still making a steady profit of $100k+ a year beyond what he needs to continue playing at this level. So he might have another amount set aside for "shot-taking" or things of this nature -- perhaps big MTT like the WSOP
or WPT. It's kind of like how a stock investor will diversify his portfolio -- you would have the rocksteady performers in your everyday portfolio (the $60k for his 600nl games) and then another amount set aside for high-risk ventures ($60k or more for bigger games/MTT).
Also, what you see at online ranking/tracking sites like PTR or OPR do not tell the whole story in terms of how much money people actually have. In addition to missed sessions, live tourneys and cash games, the pros may also have wealth from other places such as business ventures, rich relatives, or lucrative investments. A great stock broker or businessman can make MUCH more money than a great poker player could ever dream to make at the tables, and that is a fact.