Plainly a big wedge of IE came from the NE through Central Europe to the SW and displaced the farmers who had displaced the HGs. You can see it in all the DNA maps. No argument there. However there is also plainly something else that happened specifically in the northwest where I1 had a dramatic population expansion associated with Funnelbeaker in between the farmer expansion and the IE expansion.
The likely reason being (imo) that the boreal forest zone was further south at the time creating the opportunity for a zone of shifting agriculture similar to that which survived in Finland until recently (a shifting agriculture which would fit Tacitus’ description of the Germans and other evidence like the lack of permanent settlements).
current boreal zone (retreating north since the end of the LGM)
I think the southern edge of the boreal zone in 4000 BC is likely to have aligned neatly with the northern edge of LBK. The shifting agriculture and lack of permanent settlements possibly being why the IE expansion didn’t have quite as flattening an effect as it did in the LBK zone.
This doesn’t change the main argument about there being a massive IE expansion that squished LBK but as Funnelbeaker immediately preceded the IE expansion i think the two events may be connected somehow.