For example, they say "the potentially infringing or misappropriated content available on [your] Website". Keyword there - 'potentially'.
Earlier on in the letter, they say your content 'may' infringe on theirs.
It seems their motivation for contacting you is because you expressed some displeasure in the recent acquisition and that this has motivated you to steal their IP and become a competitor. Whilst its fair for this to motivate you to become a competitor, it isn't fair to assume you would steal their IP in order to do so. "...the recent acquisition by Pluralsight of Smarterer and your displeasure at the subsequent change in services and products offered by Smarterer, the extent and context of your activities establish that any infringement or misappropriation is willful..."
They have only asked to remove infringing content - not to shut down. "rights. Pluralsight is hopeful that the similarities between the Website and Pluralsight’s Intellectual Property were inadvertent and that you were unaware of the obligations placed on you by agreeing to Smarterer’s Terms and Conditions. Under the circumstances, Pluralsight would like to resolve this matter amicably."
Therefore, I would politely invite them to clarify which of your content they believe infringes on their IP, and make sure you inform them that by inviting them to clarify what content is infringing on their IP, that you aren't admitting that it is indeed infringing.