Yup, I was told the same thing and didn't believe it, so I asked a client who is a criminal defense attorney. He told me the law as argued is that its illegal to transport a firearm with a loaded magazine, period. There is no written distinction between the magazine being detachable or not. DAs can (and apparently have) said that having a loaded magazine means the gun can be made ready to fire simply inserting it and chambering a round.
So yeah, when I transport my gun the mags are unloaded, its locked up and all ammo is in a seperate section of the car.
I would strongly urge anyone transporting a firearm in MD to error not on the side of caution, but on the side of overt legal paranoia when it comes to compliance with gun laws.