bi-directional motor control using two mosfets
How is the board able to achive this with just two mosfets per motor?

And

Looking at the board I spotted two FDS4953 Dual P-Channel mosfets

Right on the front page is the hint to how it does this: -

The Dual DC Motor Driver 4A V3 allows to indipendently drive two DC
motors, controlling both velocity and direction. The minimum supply
voltage allowed is 7V, so you can use also two-cell LiPo batteries
(7.4V) that grant small dimensions and low weight characteristics.

Wrong voltage corrected below
Ignoring the spelling (I'm certainly no expert), it suggests using two 3.7 volt batteries in series and this means that one motor wire can terminate to the battery midpoint. This means that one MOSFET can pull the remaining motor wire up to 7.4 volts (applying +3.7 volts to get forward motion) whilst the other MOSFET can pull it down to 0 volts (applying -3.7 volts to get reverse direction): -