@Stevejf
If you have read thoroughly through that thread, you would've noticed the following note:
" Planned*** : Kindly note that Planned is not a guarantee, as the plan can be adjusted or changed. "
Thus, I can not. No one can speculate on this subject except the TP-Link representatives.
Keep in mind that RE300 is with EOL status, which means, it won't support EasyMesh feature, but OneMesh.
OneMesh doesn't nor will allow to be built with a OneMesh capable router in AP mode - OneMesh FAQ - Q8/A8.
So you should use some of the EasyMesh compatible devices to configure this when it's already available.
Or you could try the following:
Configure C80 in 'manual' AP mode - use this guide - Case 2. Then build OneMesh with RE300 and check if that works for you.