Data security has more aspects than merely secrecy. Perhaps you know about the InfoSec CIA triad. Confidentiality (which you seem to refer to as security) is just one of its aspects. Integrity and availability are the other two and a good backup system is necessary to ensure data availability. IOW, a decent back up is part of data security, not a compromise in it.
I agree that Qubes backup system is quite inefficient. That’s why I also asked about Bacula in another thread but it seems that got buried.