I will meet you half way. The line between base load and backup capacity is semantic. Most modern power plants (including nuclear) can ramp up and down production depending on demand. At the high elasticity end is gas generation which can ramp down to 0%, and at the low elasticity end are old nuclear reactors which might only be able to modulate 10-40%.

As you infer, it doesn't really matter whether you read my comment as base load or backup capacity, and I agree with the rest of your comment. Though I would have my own nit pick with "exceptional circumstances." Some base/backup generation is required in almost every modern grid almost every day. That's not exceptional. That's normal operation.