For what it's worth, the author mentioned that CSS Flexbox would also work, but the market share of IE 9 was too high at the time for him to use. That was 2014, and now the market share of IE (especially IE 9) is drastically lower.
it's really something that after all these years, in 2017, microsoft browsers still find a way to hold back web features.
As for K-12, is that really a requirement in some places, or is it just the case that some clients just happen to not be updated?
More generally IE9 and IE10 are both still supported by Microsoft (for Vista SP2, Server 2008 SP, Server 2008 IA64; and for Server 2012, and Embedded 8 Standard), so it seems entirely plausible Microsoft would sell security updates for other OSes to clients willing to pay for them; they definitely do that for some clients (some still get security updates for XP!).