While the European Commission is still investigating, Ukraine's sanctioning of Lukoil does not threaten the wider transit of oil to Hungary and Slovakia. They have other suppliers available to them — something they agreed to seek out when they were initially granted the exemption on Russian oil imports.
By rejecting the initial request and calling for more information, the European Commission is simply playing for time. The EU is taking these actions against Hungary and Slovakia because they oppose its pro-war policies, seeking to find peace rather than extend the conflict.