One, you kind of included the source right here. You can equip everything because weapon costs haven't been balanced yet. When the points are balanced, you won't be able to afford "pretty much everything".

Also, not to sound like a douche, but I'm not going to spend an hour looking for a source that straight out says it more clearly. All I will say is I know I've heard/read it before either on a twitch, on these forums, or on the discord from a dev. That, and ask pretty much anyone, and they all will say it's being balanced on loadout point costs not a tier system. And, if you know anything about balance, it's pretty easy to see they are balancing items around the LP cost system instead of a tier system. Especially considering they have never said, anywhere, that weapons will be restricted on a tier system.

I'm sure you could put in a question to the devs for a twitch and they will answer you flat out. I mean, who knows I could be wrong, I doubt it, but I could be. Just make the question something simple like, " Will weapons be restricted by loadout points on a class, or will they be on a tier system where only higher rank characters can have plasma/exarch weapons/melta/etc?"

Even if they implement a tier system, it won't mean much.

I mean, you will be matched with players of (generally) the same level. So once you reach the, for example, plasma tier, you will still have everyone running around with a heavy plasma.

Also, there is inflation. Unless new players keep joining the game at a large and constant rate, stronger weapons will become less scarce. This is because people constantly gain experience (and thus access to stronger weapons) which accumulates. So, for example, after some months, when the stream of new players slows down, most people will have unlocked everything.