Does frozen broccoli need to be cooked to be eaten safely?
I cannot comment directly on the broccoli, but I want to point out that this part 


  How big a risk am I taking by just blending frozen broccoli into my smoothies without cooking at all? How likely is it that I get sick and what might be the symptoms? 


is impossible to answer. This is not how food safety works. 

Creating a prediction for such a risk is as complicated as creating a weather prediction. Only the people who'd have the resources to do such a prediction for any given piece of food (for example the FDA) don't have enough data to make the prediction. They'd need to know the history of the pack of broccoli to calculate the risk. 

Instead of calculating the risk for every type of food under every condition, they calculate a range of conditions which can be considered "safe" and promise you that nothing will happen when you eat the food. All other ranges are called "unsafe", but a better word would be "undefined". The risk for them could be anywhere from 0% to 100%, and nobody can give you a better answer. 

There is no general food safety guideline that produce has to be cooked through, and normally, eating raw broccoli would be considered safe. Of course, there are still rare cases where safe food, including produce, does cause problems - we are talking low risk here, not zero risk - and there could be a lawyer who decided to wash their hands of the matter. Or they could indeed know that their food has a higher risk factor than typical broccoli, maybe they use maize-fed cow dung for fertilizer on the broccoli (which is a source of E. coli), and decided to write it there. Whichever it is, nobody can tell you but the producer. If you decide to eat it, you are on your own in that pesky "unsafe" zone where the risk can have any value. 

As long as we don't know which microorganism they suspect in their broccoli, we cannot say what amount of longer cooking at somewhat lower temperature is an adequate substitution for reaching the temperature they recommend. So that part is also unanswerable.