The lab did not take SARS-COV-2 and breed/select for lethality.
The lab looked at a virus that is related to SARS-COV-2 that was isolated back in 2017. The virus is called GX_P2V - while it's not SARS-COV-2, it is highly related. GX_P2V does not cause illness in humans. The lab claims that after some fairly typical lab actions (ie, not directed evolution or specific gain of function research), GX_P2V picked up some mutations (they call this strain GX_P2V(short_3UTR)). This strain initially showed low pathogenity in hamster models, but was later (in this study) shown to be 100% lethal to humanized mouse models (n=4).
If you believe the paper at face value, this was not gain of function research - there was no "crafting". Just a "weird lab thing happened", and then follow up experiments. The "typical lab action" here is just... growing the virus in media. They claimed that after a few passes through the grow/harvest/concentrate cycle ("passages"), that the short_3UTR variant was present.