It's definitely partly bribing and partly other reasons. Let me mention several factors.
First, let's consider that scene from CS chapter 7 itself that Sytherincess is asking about. Obviously Hermione is a biased observer, but notice how none of the Slytherin team apart from Malfoy is described to react in any way. If the claim of a bribe was so absurd, surely they'd be outraged too.
Second, let's consider how there just aren't enough good Quidditch players available from students in a house. Harry is an exceptionally good Seeker, and we know how the Gryffindor Quidditch team faces the lack of good players often. Even if Draco didn't match the bar of the rest of his team, there just might not have been enough Seeker types in Slytherin.
Next, I imagine as the head of house, Professor Snape probably has some say about what happens in the Quidditch team if there are arguments. If Draco was a really bad player, he would veto Draco playing. Recall how Professor Snape even gave detention to Harry during a Quidditch match in HBP, knowing fully that this means he couldn't play on the final match. Surely it's much more difficult to bribe Professor Snape with brooms than it is to bribe the team captain and other students.
On the other hand, Professor Snape seems to have favored Draco even among Slytherins, which might have taken part in Draco getting in the Quidditch team as much as buying new brooms.
The final factor why Draco got in might be psychological: the Slytherin team knew about how Draco playing Seeker would annoy Harry, and betted on this inducing Harry playing worse on the match.