Oracle SQL Developer -> manually alter sequence increment by 10 --> commit needed

Your alter statement does not need to be committed because it is DDL rather than DML. Losing the connection won't have any effect. But it also won't in itself increment the sequence value at all.

What it does is modify the sequence definition so that next time, and every subsequent time, nextval is called the generated number will increment by 10 rather than whatever it was set to before (the default is 1).

So as a quick demo:

CREATE SEQUENCE myCounter;

Sequence MYCOUNTER created.

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
         1

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
         2

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
         3

So the sequence is incremented by one each time, by default. Then after you alter it, each call to nextval increments by ten instead of one:

ALTER SEQUENCE myCounter INCREMENT BY 10;

Sequence MYCOUNTER altered.

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
        13

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
        23

The alter itself didn't use up or skip 10 values - otherwise the first nextval would have got 23 rather than 13. It only changed the definition.

The documentation you refer to is saying that if you roll back, you don't get any of the previously-issued numbers generated again:

ROLLBACK;

Rollback complete.

SELECT myCounter.nextval from dual;

   NEXTVAL
----------
        33

You get 33, not 23 again. A value is issued and then is no longer available for any other session, including your own, to get from another nextval call - regardless of rollback or commit, or whether the value is actually used for anything. (It may eventually be re-issued if the sequence is set to cycle, but it won't be by default).

Note also that sequence generation isn't gapless, and isn't meant to be, both because of how rollback is handled and how the cache mechanism works. The numbers also aren't always issued strictly in sequence - in a RAC environment each node has its own cache and will allocate from within those.