Make sure you don't use this for bad purposes but I will give you the way since these are public comments and anyone can view them.
First of all, it seems like that "TsssKhh" and "Sasha Morra" are the same acount on YouTube and the only reason that "Sasha Morra" gets more results in google by "Sasha Morra" site:youtube.com is that it's the current screen name and "TsssKhh" doesn't exist any more except in links and it explains also how this user is active in comments (as you mentioned) but no results came in google. Now the results for "Sasha Morra" site:youtube.com works just fine but why there are only 3 comment video links in the results?? My best guess is that most of the comments have the older screen name "TsssKhh" which doesn't exist and thats why Google can't find those old comments.
If you are searching for "Sasha Morra" aka "TsssKhh" comments specifically then I can't help you but if you were giving her as an example only then you are ok because I've tested "User" site:youtube.com on an active user and it gave me 9 pages of results up to 2 years range all of links to YouTube videos that he/she commented on except 2 links to his/her channel.
How I did it simply copy or type "User" site:youtube.com on Google you might encounter three type of results you can know exactly which one by looking at the grey text under the links of the results;
This picture below show the 3 types of links.
Video links that he/she commented on (Red underline: user,Yellow: Number+Time, Blue: The comment)
see under how to watch all comments in the link
Channel links that are subscribed to him/her (Red underline: user, Green: Channel........)
A links to videos that he/she watched (Red underline: user ,Purple: Number+vies)
Open the links and find the comments that you want if it's a video with few comments you will sight the comment easily if not then what you need to do is following the picture
paste user in the top right box if the 3 steps doesn't work load as many comments you can and then repeat the 3 steps.