You are right it will make a difference but for importers such as EU not exporters such as Russia.

It's pretty simple, it's a matter of supply and demand and the fact that neither of them have changed.
Just because EU doesn't [supposedly] import Russian oil it doesn't mean they don't need oil anymore. Instead it means EU is going to buy it from elsewhere. But since the production has not increased anywhere else (ie. OPEC) that means if EU is importing oil from lets say Saudis they have to replace one of Saudi customers like China (just an example) but that also doesn't mean China won't need oil; instead that means that now Russian oil is going to Saudi customers that were replaced by EU (China in this hypothetical example).
In simple terms this means if Russia was exporting for example 1 million barrels of oil, they are still exporting 1 million barrens of oil but to different customers.

So why is it screwing EU and helping Russia?
That is pretty simple too. EU is now buying oil at a much higher price (almost $120) compared to before ($40-$60) so EU is spending a lot more money while the increased prices also cause inflation in those countries.
On the other hand Russia is selling the same amount of oil for a much higher price which means their revenue is almost doubled after the "embargo".

On top of that, China (the biggest economy in the world), is going to receive a small discount on the same oil which would put their economy further ahead. Their products will be cheaper than what is produced in EU so they would take over more of world's economy while replacing EU customers. This would shrink EU's GDP...

P.S. Any comment on Germany gas being cut off and how they were predicting a $240 billion hit and a "deep recession" in Germany as Reuters put it a couple of months ago?