That's just jls being jls, nobody understands him when he's ranting...
The arrest wasn't "illegal", in that every arrest is reviewed by prosecutors and if the prosecutor determines the arrest was "proper", they proceed with charging the arrestee with appropriate charges for the crime(s) the arrestee is alleged to have committed. If the prosecutor determines the arrest was not proper, they will void the arrest and release the arrestee if there are no other reasons to hold him.
Running from the cops is not an arrestable offense in and of itself, what it does is gives the cop a reasonable suspicion and allows them to stop and question you. Once a cop does stop you to question you, he is permitted to do a patdown to see if you have a concealed weapon for his own safety. The cops in Baltimore arrested Gray after having found the knife under the mistaken belief that the knife was illegal to carry.
The reasoning for the prosecutor saying the officers had no probable cause to arrest Gray was because the knife found wasn't illegal to carry. However, being on probation, the carrying of any weapon might be a violation of his probation.
Under normal conditions, all these things are sorted out once an arrestee gets to Central Booking, in Gray's case that didn't happen due to his tranfer to the hospital.