Why is it nonsense? Cruz is closer to a libertarian than either Trump or Rubio. Most libertarians take the Constitution seriously and do not approve of the NSA spying on us without a warrant, which both Trump and Rubio support.
Let us look at another major libertarian issue that prevents Paul supporters from going to Cruz rather than Trump - remember, Trump does not drink, smoke or is in favor of complete legalization of marijuana... however:
Trump's openly stated position - on those states having changed their marijuana laws - is that he would not send in federal authorities to enforce the federal laws. Paul is for recreational use. Rubio, backed by private prison money - said he will send federal troops into those states.
Here is Ted Cruz's position:
I will say one thing that's been dismaying about the Obama administration,... he continued. The Obama administration's approach to drug policy is to simply announce that across the country, it is going to stop enforcing certain drug laws. Now, that may or may not be a good policy, but I would suggest that should concern anyone - it should even concern libertarians who support that policy outcome - because the idea that the president simply says criminal laws that are on the books, we're going to ignore [them]. That is a very dangerous precedent.
Where one stands on the issue is not the issue here. The question is if Paul's libertarian following would go to Cruz. I think not.
Moreover, notice this quote from 'The Office of National Drug Control Policy'
Since 1996, 23 states and Washington, DC have passed laws allowing smoked marijuana to be used for a variety of medical conditions. It is important to recognize that these state marijuana laws do not change the fact that using marijuana continues to be an offense under Federal law.
That's 23 states that are in violation of federal law, right now and 23 states that could see some interesting ads if Cruz gets the nomination, which is a greater worry than getting the support of Paul's voters.