I have gone to the site. I have accessed it.
First it is not 1% but almost 6%. That you can verify by calculating the announced number of voters versus the 20.8 m ballots.
Secondly, the IEBC embarked on a rather clever mission. It adds 1% to the number of voters per polling station then rounds it off to the nearest 50 or something. The more polling stations there are (with ghost voters) the more of the nearly 1M ballots you got.
The bottom line is the tweet does not answer the concern notably the claim that only 1% was added. In reality almost 6% has been added.
Feel free to do your own counter calculations and either confirm or counter my assertions.
So there is no doubt, one could argue about how the ballots have or can be accounted for. However if you are telling people that you only added 1% and that becomes your official position, then you fail to account for 4 to 5 %. Note that 1% made all the difference in 2013