Are STO novels considered canon to the rest of the novelverse?
No. Star Trek Online: The Needs of the Many
does include a chapter referencing the novelverse as an alternate reality in the same multiverse, but it was meant as an easter egg for novelverse fans and not a definitive statement. They're really just different stories written around the canon episodes and movies, and differ in ways that a STXI-style branch point in history can't believably explain - like the ancient Iconians of STO being completely different to the Iconians of the novelverse's Gateways