Yikes. At first glance, it appears that the author is trying to show the difference between an apple and an apple. The "overcoming of the presumption" and the "plaintiff's burden" are one in the same, no?
On the other hand, he might mean to say that it is unclear whether the court applied the "mailbox rule" presumption (that mailing = receipt) or just placed the burden on the plaintiff to show receipt in the first instance.