In garrison mode, units have 50% movement and 50% offensive firepower along with +2 anti tank ... Does this mean that units in garrison mode defend at full strength since is it not technically "offensive" ?
Yes it will defend normally. Actually even better than normal because of the extra 2 guns. Guns are good in all combats, but more even more so in defense as defender guns fire before attacker guns.
But the 2 OPs can make it easy to get encircled. So very good in some situations, less so in others.