You got it.
To be technically accurate and hopefully reduce confusion, the opposing sides of a figure-8 microphone have opposite polarity not phase, but that (mis)use of the term phase is pretty common.
It's true that the opposite polarity lobes of the side mic is part of what makes M/S possible, yet the mid mic doesn't need an opposite polarity lobe- there is one in the case of M/S Blumlein or if you use a supercardioid mid mic, there isn't if you use a cardioid or omni for the mid mic.