I'm not sure that Redress has claimed them to be that, but I can wager why some on the far left would see the tea party as "radical" or "extreme".
Many Tea Partiers are willing to feel some short term discomfort for potentially beneficial long term improvements. To many, taking such a stance is a "radical" notion, akin to amputating ones leg due to severe dabilitating pain in it rather than living life full of pills. For many people opening yourself up to hard times, even if its the chance of a much better life in total, is a "radical" or "extreme" thing. Couple that with the fact that many on the far left think that the views of what the Tea Party stands for won't lead to "good" things but to bad and suddenly it seems they're pushing for bad now for bad later.
Many Tea Partiers are extremely upset with government, how it works, and the size its bloated. As with any group, hyperbole builds in at times and calls to severely reduce government comes off as an immediete goal to get rid of a huge hunk rather than something that realistically will take time to do. Our society since the New Deal has been one with varying large degrees of government involvement in peoples lives and the notion of moving significantly away from the norm can be seen as "radical" or "extreme".
They could be rather "extreme" or "radical" themselves and, as is typical, they view the Tea Party as sort of a mirror, focusing on the fringe elements and extrapolating them as the norm because it suits their political agenda. So they see the fringe that is building up compounds to try and refuse the government, citing "right wing terrorism" yet would never look at groups like ELF as the general representation of environmentalism. They see a fringe amount of klan members supporting the movement and cite that its a racist group, while ignoring the elements of MS-13 and La Raza in the open-borders movement.
Ultimately the Tea Party is a solidly right wing fiscal and governmental organization. To my understanding I would not call it "extreme" or "radical" in a literal sense....fiscal and governmental extremism on the right would be the abolishment of almost all taxes and absolutely hardlined limiting of what government can do to the most narrow literalistic reading of the constitution, which is not the case for the majority of tea partiers with one step away from an anarchy being the form of government. Now, comparing the Tea Party's views to modern politics and generally what we've seen from both parties over the past 20 to 30 years...yes, it could be seen as "radical" and "extreme" as neither side on a national stage have showed anywhere near the desire nor gumption to seriously and legitimately work toward significant cuts to spending. So in the sense of being extremely different than the norm, sure. And comparitive to a far left liberal view, it could easily be seen as "extreme" as its significantly different than their views.
But if you ask me if its "extreme" comparitive to a legitimate, honest, moderate (and not a staunch liberal who likes to call themselves a moderate by acting like everyone thinks like them because its common sense and anyone else must be some extreme crazy righty)? No, its staunchly to the right, but its not extreme from a point where you start in the legitimate middle.