Ground kills. How they were credited to LW fighter pilots?

[Dénes Bernád]

That's a very good question.
AFAIK, ground 'kills' were not officially counted as air victories for Luftwaffe fighter pilots (by contrast, they were in ARR). On the other hand, destroyed tanks were counted in the Schlacht pilots' tally.

Referring to the particular incident you mentioned, it's quite plausible the Luftwaffe pilots reported enemy aircraft destroyed on the ground as "shot down while flying at low level", or "while taking off/landing"...