Description above says that Octavo Systems OSD3358 include ARM Cortex-M3. Is it true or simple typo mistake?
It is simply ture. It is inside the AM3358 processor which is inside the OSD3358. You can look on page 1213 of the Technical Reference Manual for the processor. http://www.ti.com/lit/ug/spruh73p/spruh73p.pdf
However, it is not much use as it is dedicated to power management functions (PRCM) for the processor. So one could argue, that while it Is there, it is not really there as you cannot use it for other things.