So, it impossible to generate the heat the sun does with the use of fusion?
Your question is a bit confusing. I will assume that you meant to say "without the use of fusion".
From my understanding, it is impossible for hydrogen to NOT fuse at the center of the Sun given the temperatures and pressures. There is also the fact that we observe
neutrinos escaping the Sun which strongly evidences fusion reactions.
To bring this back on topic, we don't have to directly witness the fusion reactions in the middle of the Sun to know that they are happening. We can use observations to infer that fusion reactions are occuring.