May be wrong but I'm fairly sure that Yanmar are one of the few manufacturers like Sole that design and manufacture engines for marine use and not just marinise base models.
Yes it is a Yanmar designed and made engine. Larger Yanmars are also based on Toyota engines.
BTW don't think Sole make their own engines any more. The small ones are marinised industrial engines - Mitsubishi from memory. Apart from Yanmar, just about all small marine engines are based on industrial engines, usually Japanese and Yanmar also use their own marine engines in industrial and agricultural applications.