Once malware infects a system, it looks to stay there even after interruptions that might cut off its access. This behavior is known as persistence. The underlying details of how the malware achieves persistence is known as the malware's persistence mechanism.
For example, without boot persistence, the malware would simply die and cease to function after a computer reboot which would kill the malware process, so the malware needs a way to automatically start up each time the computer is turned on. That way, the malware doesn't require the user to explicitly run the malicious file each time to re-infect the system.
To achieve this, malware oftentimes uses the same exact techniques that legitimate software does when it wants to automatically start up (software such as software updaters, notifiers, services, drivers, etc.). The most common technique would be to create a run key that references the malicious executable.
RunOnce registry keys cause programs to run each time that a
user logs on. The data value for a key is a command line. Register
programs to run by adding entries of the form
description-string=commandline. You can write multiple entries under a
key. If more than one program is registered under any particular key,
the order in which those programs run is indeterminate.
The Windows registry includes the following four keys:
Keep in mind, there are other ways of achieving boot persistence, and there are many other run key locations, some of which require more privileges and are system-wide. With administrator privileges, the malware could register itself in the HKLM hive rather than the HKCU hive, so the malware could automatically start up each time regardless of the user logging in.
Anyway, to answer your exact question, when the source says that Dexphot utilizes "redundant boot persistence mechanisms", they most likely mean that the malware has multiple boot persistence mechanisms that act as fail-safes, so even if one of the persistence mechanisms fails or stops working for whatever reason (e.g., a run key referencing the malware is deleted), the malware can still remain persistent and automatically start up when you reboot your computer.