Using $wpdb->insert and or related methods to modify data within any of the default WordPress tables, be it posts, postmeta, user, usermeta etc is discouraged because there are functions which already exist for the purpose of modififying data within those tables.
For example,
Database Queries
Avoid touching the database directly. If there is a defined function that can get the data you need, use it. Database abstraction (using functions instead of queries) helps keep your code forward-compatible and, in cases where results are cached in memory, it can be many times faster.
https://make.wordpress.org/core/handbook/best-practices/coding-standards/php/#database-queries
However it is acceptable to use the wpdb class on custom tables for which no function wrappers exist for the purpose in which you need.
You can try adding a comment on the line of the query:
$wpdb->insert() //db call ok
$wpdb->insert() //db call ok; no-cache ok