EDIT: But back to the question. I think the answers make sense. It's easy to bypass if you are savvy (as we know already) but if you challenge govt they would take offense. I guess the grey area would be anonomous critique, but I digress; thanks for the response.
So in praising Tang Taizong, the official was critiquing Xi's inability to deal with differing opinions.
This isn't a new thing, BTW. Chinese scholar-officials have been doing this for ages. China has historically been a very authoritarian country and the same ideas applied then (kind of). This is one reason why historical Chinese officials were (ironically enough) criticized by the Communists for being backwards looking. Straight forward critique wasn't allowed during the imperial eras so they always had to find historical parallels to vail their critiques and advices.