Is it true that Marketing and Sales activities are a part of Business Development?

Yes a signifiant part.

Marketing is a vital part of business development as a business development manager cannot develop or build business without carrying a marketing message (without being the one directly in charge of it)!

Sales is the ultimate translation of such efforts into tangible results (without being the one directly in charge of them) Inbetween there are many other processes involved that make a business development manager the link between marketing & sales.

Business development plans might include marketing and sales activities and might not. They might relate only to developping your business processes.