https://mariadb.com/kb/en/library/instant-add-column-for-innodb/ states in the Limitations section:
"If the table becomes empty (either via TRUNCATE or DELETE), the table will be converted to the old "non-instant" format."
The fact that this is listed as a limitation, and the fact that it makes reference to 'the old "non-instant" format' both imply some sort of limitation. This leads users to understand that the instant add column ability is being lost in cases where the documentation states the table is being converted to "the old format".
Explaining that columns added 'instantly' are stored in separate structures associated with the table like BLOB columns, and operations like emptying the table take the opportunity to flatten out the table definition, incorporating the instantly added columns into the main table structure, without referring to it as "the old non-instant format", would avoid the misunderstanding.