The way I always understood it, a blitz required sending more people than the offense had blockers for. That being the case, sending five guys at the quarterback is NOT a blitz, whether two of them are linebackers or safeties. There are five offensive linemen, whose only mission in life during passing downs is to protect the quarterback. On passing downs, there's also a running back, and maybe a fullback or a tight end to help with the blocking. Again, there may be a nuance I don't understand here, but I'm pretty sure I'm right on this.
There may be some terminology somewhere that makes rushing five guys a blitz, but I don't remember there being some unwritten rule that you have to drop seven or eight guys into coverage just because it's pre-season. When a defense is set up so that guys are supposed to react to situations, I'm not sure where anyone expects a linebacker to just sit there and wait for the quarterback to throw the ball over his head.