Where does it say that?
Purism may well provide “secure boot” but that is not the same thing as “[UEFI] Secure Boot”, which is specifically what the article linked in the OP is referencing.
Up until recently, as I understand it, PureOS didn’t even support UEFI, which in practice is a pre-requisite for using “Secure Boot”.
In practice, yes.
However if you freeze your system at a point in time and you make the bold assumption that at that point in time your system is secure (so that your security is no longer dependent on Purism) then both Secure Boot and PureBoot aim to solve the same problem: Has my system subsequently been tampered with?
In other words, has my system that was previously secure been tampered with - so that it might now no longer be secure?
If it wasn’t secure to begin with then it doesn’t so much matter whether tampering has occurred. Indeed, knowledge that no tampering has occurred might lead you into a false sense of security if the system wasn’t secure to begin with.
Of course we all know that in practice the system “as frozen” was unlikely to have been secure i.e. there probably were security bugs, we just didn’t know about them. Perfect security is, at the moment, an ideal. Ironically, governments would hate perfect security.