ELI5: What do Chinese economists argue are the benefits of not floating the Renminbi without restrictions?

Well, realistically, Chinese economists don't "argue" anything. The government has set the rule that this is the case, with the goal being to artificially keep the currency lower value to increase exports. The low currency allows China to maintain a strong position in the world market for exports, and keep their costs low overall to operate businesses in China.

This comes at the negative effects of people being paid in renminbi (i.e. Chinese people), but China straight up doesn't give a F about their people and are more concerned with growing China's market and economy overall and getting a dominant position for export goods, at nearly any cost. And obviously being in China, those who may question this tactic just sorta disappear.

The issue is the "no restriction" part. Most countries have a "managed float" system and are not completely restrictionless. However, a float implies a certain degree of freedom in buying and selling that currency in an open market. This has the danger of excessive short term capital inflows and outflows. This also has the danger of being more open to speculative behavior by foreign exchange investors (speculation in one person's view is considered a rational expectation from the perspective of another)

For China in particular, there are probably two main concerns 1) rapid changes in the value of the renminbi (which can be damaging to businesses) and 2) possible capital flight from Chinese people who wish to invest overseas. China stands accused of keeping their currency low which means imported goods are expensive for Chinese residents but benefit their manufacturers who focus on exports.

Also the mentality of the Chinese government plays a huge role. It is very top down, centralized and their sense is that anything vaguely "chinese" must be under their strict control. A floating currency runs very much against that overall mentality.