So a lab in fact has created a highly contagious flu virus (far higher than regular flu) through gain of function research which has indeed killed 100% of the subjects which contracted it even if they were only mice.
This paper does not appear to support that. They indicate the source was an existing, in the wild virus.
>> We isolated an HPAI H5N1 clade 2.3.4.4b virus from wild birds in Korea with 96% E and 4% K at amino acid position 627 of PB2. To investigate the genomic characteristics of this clade regarding mammalian adaptation, we studied the replication and transmission of the H5N1 virus in mice.
They infected mice with a wild strain that already was pre-positioned to better infect mammals used super high viral loads for that then watched as the mutations which promote mammalian infection evolved and exposed challenge mice to the newly developed strain.