Err… that was probably a typo; what he meant was K26. As in linux kernel, version 2.6. The story is in the second post of that thread he linked to.
TomatoUSB depends on Broadcom's (more or less) closed-source wireless drivers; the E4200 uses a new chip — BCM4331 — for the 5GHz radio that isn't supported by any available kernel 2.6 drivers.
Linksys's own firmware and GPL release are still based on kernel 2.4, so the module they use won't work, unless they're added to the K24 branch of TomatoUSB, but that would require making a whole lot of OTHER changes to the K24 branch to get it to run on the E4200, and putting this work into improving K24 (which is old and broken in many other ways) seems like a step backwards and a poor use of time.
The general hope/expectation is that soon or later some other brand that are using kernel 2.6 in their stock firmwares (eg Asus, Netgear) will release a new model that also uses the newer chip, at which point we'll be able to grab the driver from there.
Can't guess at a real timeline for this, since Asus (for one) is in the nasty habit of announcing new models months and months in advance with no details on actual availability, and there's no way to really know if a given model uses BCM4331 (or if its drivers support it) until it's actually in someone's hands. So the best anyone can do for now is speculate.