Yes, if you have an even worldwide visitor share. Can I Use[0] puts USA IE9 usage at 3.86% and IE8 at 7.28% (11.14% in total). If your user base is different it would be wise to examine the data before discounting flex-box based on worldwide stats.
If anyone has access to netmarketshare's USA[1] (used in the article) data, it would be nice to see what it is.
EDIT: Also the article's stats[2] are for desktop only. It seems[2] that mobile support is relatively high with Can I Use[0] stating that total USA support = 86.46%.
[0] http://caniuse.com/#search=flexbox [1] http://www.netmarketshare.com/browser-market-share.aspx?qpri... [2] http://www.netmarketshare.com/browser-market-share.aspx?qpri...