I am not aware of a method to only update records without creating new records where there is no match using the CSV import app. You could run the app, and then manually delete any newly created records. You could easily identify the new records by looking at the created time.
You could automate this if you spend some time setting this up properly.
You would first need to use the CSV app to import your records into a completely different table, and then use an automation that triggers whenever a new record is created in this table.
This automation would compare each record with the records that already exist in your original table. If there’s a match, then update the appropriate fields.
You could use Airtable’s new conditional automations to handle this for you. Note that this would only work if you’re sure that there would never be more than ONE matching record in your original table.
p.s. Also, I would send an email to firstname.lastname@example.org to request that they add your requested feature into the CSV app.