Remember, when we abbreviate an ipv6 address all leading zeros are reduced to a single 0.

E.g

0003 would just become 03

When there are geoups of 4 zeros these can be represented as a single 0 or as a double colon ::

But we can only use the :: once so when summarizing an address containing multiple groups of 4 0s one after the other they can all be abbreviated to a single ::

Eg

fe80:0000:0000:0000:0210:5aff:feaa:20a2 would become fe80::210:5aff:feaa:20a2

Therefore it is perfectly valid to abbreviate an address of 0000:0000:0000:0000:0000:0000:0000:0000 /0 to just ::/0