Your way of playing a minor 7th chord is OK; I've seen several experienced guitar players use it. The other way shown in the picture may be more common, and the reason is that such a minor seventh chord is often followed by a 9th chord a perfect fourth above it, e.g., Cm7 - F9, and that F9 chord is usually played like this (from low to high E):
X 8 7 8 8 8
where you put your 2nd finger on the A string, your first finger on the D string, and your third finger on the top 3 strings. So if you come from a Cm7 played with your 2nd and 3rd finger, you just shift the grip down by one string and add your first finger. This can be done very efficiently, and it requires much less hand movement than when you use the thumb and the first finger to play that Cm7 chord.
But, as usually, correct is whatever sounds good and works for you.