That's definitely not true. I paid $10 for this game last year and I have been able to download every newly released update since then.
If someone reading this is having trouble trying to access the new update, then check your collection page. You own the games that you paid for.
Also, a one-time purchase just means that you pay once for the game and receive every update at the price you paid, not whatever you just said.
So, some developers on this website will release updates for their games in paid tiers. Say for example that they decided to price it at $10 for the first two weeks after release. Then they decide to lower the pricing down to $5, which might be the situation you might currently be in.
Without spending any further money, you would then have to wait for them to drop the price down to the initial offer you bought the game at, $5, in order to play the game earlier than the public version.
