Of course Jeremy Corbyn and John McDonnell do not want to be subject to the EU's rules against capital controls and State Aid. The 2017 Labour manifesto was wholly incompatible with those rules.
The Labour internal market amendment is just a device for ensuring that the Lords one on the EEA does not pass. Corbyn has been in the House of Commons for 35 years. He knows how to do things like this.
And he has succeeded: the EEA amendment will not now pass.