In this they are correct: Outside of fusions and log in champs (though these are not shards) and summoning events where you get a specific champion after opening a certain amount of a specific shard type, there are no guarantees on getting missing or specific champions from shards. 

Are you asking if the odds of you getting specific champions change based off of the ones you already have? If so, the answer is no. Each pull is 100% independent from each other, and the only thing which influences your odds of getting specific champions are the various summon events that happen - for example:

• Right now, your odds of getting the legendary champ Warlord are increased by a factor of 10 when pulling void shards. Your odds of getting a legendary champ are unchanged. 
• This weekend, there will be an event where your odds of getting a legendary champ from an ancient shard are doubled, from 0.5% to 1%.

*edit - damnit! Thorne you actually beat me for once! I blame the slight amount of time it took me to re-edit my post to add bullets -_-

This is a glorious day. 🙌

Sigh..... will have to save my mocking of you for another post, Thorne. :D

🤣

Also want to include here that after 200 ancient or void shard pulls without summoning a legendary, mercy comes into play and your chances of pulling a legendary increase each summon until you are as close to 100% guarantee as needed in order for RNG to give you a legendary. So i suppose this would essentially improve your odds of getting a new legendary since you are at greater odds of getting any legendary during this time. Same with Epics, but the chances start to increase after 20 pulls without summoning an epic champion. For Sacred shards ,mercy comes into play after 12 pulls without Legendary. 

In case you didnt know that all of this information was available in game, at the summoning portal. 🤷‍♂️

I only started a week ago and the one Purple shard I got gave me a blue, yet I got three purple Champions and a gold Champion from like 15 Blue shards, lol. RNG be weird sometimes.