This is an interstitial ad. In general, they're defined via cookie to not display every single time you visit the site, though the interval is arbitrary. In the case of Forbes, it should only display about once per day, per browser, but some sites opt for every few hours. The practice is common enough that if there were a significant SEO penalty known to come about because of it, you'd assume people would stop doing it. Note in particular that this type of ad is mostly restricted to larger media sites that would absolutely not tolerate a problem caused by it.
But if the case is that the particular bits of JS relevant to the interstitial do not get executed, the question is moot as Googlebot just never sees the ad. This is easy enough to test by just disabling JS in your browser.