"I been told that the heat inside the compost is not high enough to kill the seeds even it up to 65 degree Celsius or even higher, Is that true?"

No. Hot composting (140-160F) for at least 3 weeks will kill it. But that requires substantial compost management. The best solution is not to allow the plants to grow to seed begin with and you can do that while the plants are still young by cultivation or manual removal.

Organic herbicides will have only modest effectiveness, but those containing clove oil (eugenol) or acetic acid will work better than most. They kill topgrowth by desiccation, so need to be applied in sunlight and when the plant is in active growth. And again, younger plans are more vulnerable than mature ones. They are also non-selective so keep off of any crop plants or other desirable plants.

Thanks for the advise. I have tried using the acetic acid (vinegar) to spray onto the pigweed, but it will lower my soil pH (acidic). And the effect of killing pigweed is not that significant. I never try clove oil before, will give it a try. May I know any recommendation rate of application for the clove oil?

Currently, my compost management is: (1) Mix all the necessary materials together; (2) Bag them equally in gunny sack; (3) Stack them into a big concrete pit; (4) Keep them about 3-4 week and use. The heat is there, just that may not last long, like the first week heat up, then slowly go down, third week the temperature get rise and heat up again.