The following extract form Wikipedia seems to summarise my doubts:
" MMT argues that as long as there is a demand for the issuer's currency, whether the bond holder is foreign or not, governments can never be insolvent when the debt obligations are in their own currency; this is because the government is not constrained in creating its own fiat currency (although the bond holder may affect the exchange rate by converting to local currency).
MMT does agree with mainstream economics, that debt denominated in a foreign currency certainly is a fiscal risk to governments, since the indebted government cannot create foreign currency. In this case the only way the government can sustainably repay its foreign debt is to ensure that its currency is continually in high demand by foreigners over the period that it wishes to repay the debt – an exchange rate collapse would potentially multiply the debt many times over asymptotically, making it impossible to repay. In that case, the government can default, or attempt to shift to an export-led strategy or raise interest rates to attract foreign investment in the currency. Either one has a negative effect on the economy. "
The NIIP seems important in that it calculates the ratio of foreign debt to assets. Many major countries have far larger foreign debts that foreign assets, making them vulnerable to printing money. If (when ?) the USA loses its status as world currency they may also have to deal with their astronomical debt.
I suspect that countries receiving loans under the Belt and Road have negative NIIP's and as such will be "at the mercy" of China when they fail to repay. I suspect that this is a deliberate strategy of the Chinese government.
So MMT sounds like it supports a strategy for countries with low foreign debt.
An analogy I thought of:
Gambling in Australia moves money around within the country. individuals get better or worse off, but the countries wealth is unaffected. A gambler from another country either wins wealth away from Australia or loses wealth to Australia. When they leave, the country is materially richer or poorer.