My personal opinion is that as ‘Crosses’ were not yet being widely used within Palestine by Jewish-Christian, I see no reason to assume that a ‘Crusader-era’ Cross would a) exist during the 3rd century CE, or b) be transported to China as an ‘act of faith’ and ‘conversion’! All these (hideous) aspects of Christian belief would not infest the teachings of Christ for many centuries to come. As I have said elsewhere, even the 8th century (Nestorian) Christian (stone) monuments preserved within China do NOT possess a single example of a ‘Cross’!
The ‘Couplet’ was probably composed during the 14th century CE to placate and interest the ‘new’ Sun Wu Emperor – a peasant who had overthrown the (foreign) Yuan Dynasty and re-established Chinese rule of China! This Emperor apparently liked poetry and particularly favoured ‘Couplets’ – so perhaps an over-zealous Minister (who had heard of ‘Christianity’) decided to fabricate the ‘finding’ of a ‘Cross’ (also constructed during the 14th century) – which soon had to go ‘missing’ so as to prevent any serious investigation of the matter – and the entire fabrication being found out! Heads would have rolled!