“When you tweak the EQ bands, all the internal mathematics will happen on the 64 bit range, and the resulting values will be rounded to the closest 16 bit values.”
“It is a total myth thinking that plugins will increase the actual bit depth on input, then lower it back to original on output.”
No. The math will happen in the 64 bit range, but the result will not be truncated to lower bit depth. The result will be outpus in 32 or 64 bit float format, depending on your Cubase setting. This is true for any audio processor, even if it is only a gain knob which is not at the neutral position.