The phrase I/ I'll bet you do, means I really believe you, so much that I'm willing to make a bet on it. The scenario in which it's stated, however, is what actually determines the meaning.
Bet means okay, and is often used to agree with someone. For example, if someone asked, Can we go to the pool tomorrow afternoon? their friend might reply with Bet!
The variety's name was initially UEB 32642 when it was introduced in 1999, and over time, it was rebranded to Opal apples. After their release, Opal apples were further studied in over 63 orchards across Europe and were introduced to the United States in 2010.
According to the official opal apple webpage, the apples are a cross between Golden Delicious and Topaz. They were discovered in 1999, tested in 63 orchards throughout Europe and the Mediterranean for 12 years, and introduced to the US market in 2010.