These are reasons why it might not be the largest provider of funds per capita, not why it would be by orders of magnitude the biggest recipient.
I have been to Luxembourg and to Hungary, Bulgaria & Greece - the otherwise obvious contenders for "poorest" in the EU and Luxembourg should not be in the picture.
If it gets funds for restoring one railway bridge or something of that sort the fact the population is tiny makes the per capita investment look huge, just usual tiny country effects.
A bunch of foreign companies also incorporate their EU subsidiaries there (presumably due to some tax benefit). I imagine that distorts their GDP quite badly as well.
I presume this is because of the EU institutions there and that expenditure to maintain those institutions counts towards receipts (and this effect is then exaggerated due to Luxembourg's small population). Certainly no one in the EU is under any illusion that Luxembourg is poor, much less vastly poorer than the next poorest EU country.
Notoriously difficult to portray correctly in EU money-shuffling statistics. Some money not granted to the grand duchy still filed under "beneficiary country: Luxembourg" due to some program or institution being headquartered there. And it is essentially impossible to compare apples to apples what happens in actual EU budget and what happens in Kirchberg, home to EIB.
Additionally a lot of the EU's institutions are based there or have offices there, some of which might count as investments as well.
Lastly, everything there is really expensive. So you need to invest a larger amount to achieve the same thing as elsewhere.