Did Thumamah prevent the people of Makkah from receiving provisions, even though the Messenger of Allah (peace be upon him) had written to him to allow them to transport supplies to them?

The general principle in dealing with disbelievers is permissibility within the framework of legitimate governance, and this is determined by benefit (maslaha). If benefit is found, then that is Allah's law, and it is only prohibited if a predominant harm results. Economic boycott predominantly achieves a significant benefit; therefore, scholars have issued fatwas permitting it, and the determination of this benefit is referred to them. The use of the hadith (definition) of Thumamah (may Allah be pleased with him) as evidence for economic boycott is valid because the Prophet (peace be upon him) approved his principle of boycott. And when the matter was referred back to the Messenger of Allah (peace be upon him), and Quraysh wrote to him, imploring him for permission to transport food, the objective of the boycott was achieved, which was to humble the enemy and glorify Islam. Perhaps other benefits appeared to the Prophet (peace be upon him), such as winning over their hearts.