Technically yes, you potentially do literally have 300mg testosterone "in your system".
But, the big qualifier is *where* in your system is it? Well, immediately following your injection, 200mg (approx) is sitting as a depot at the inj site. Not much use to you there, yet. So its in your system, yes, but not yet distributed via plasma.
Its a bit like eating some food. Yes, you now have 55gm protein "in your system"...but until that food is digested it is not yet distributed through your system, but "sitting" at one site, your gut.