the vulnerability was that due to a bug in the Monero code was credited 100 times more than pending
details can be read here (on russian only):
forklog.com/kriptobirzha-livecoin-poteryala-svyshe-1-8-mln-iz-za-uyazvimosti-v-kode-monero
According to your link, situation is strange as well funny. The bug you mentioned is not classic critical bug. It looks like a small transient bug appeared on a short time period, possibly due to protocol changes (hint: simple x10 amounts). Monero devs seem to be correct - they emphasize in the reply to Livecoin, one MUST NOT ignore software updates.
Another funny thing is that the bug appeared around April 6 hard fork, the article mentioned some Monero online web block explorers stopped to update the block height. How could Livecoin omit the whole hard fork?..
Sure, if the bug were critical, MOST of exchanges should be affected. But they aren't. So, make your own conclusions about Livecoin.
Just to add: if a bug is CRITICAL, devs should NOT broadcast it BEFORE FIX to the whole Internet as Livecoin considers.
Additional article from that site says Monero devs did fix several vulnerabilities during HackerOne event very quickly:
https://forklog.com/v-kode-kriptovalyuty-monero-obnaruzheno-neskolko-uyazvimostej/