Canadian Taxes: Can I claim the full flat rate (currently $23) for a meal even if the meal costs less?

This is from the exact source you quoted: Meal and vehicle rates used to calculate travel expenses for 2020 and previous years

Meal expenses

If you choose the detailed method to calculate meal expenses, you must keep your receipts and claim the actual amount that you spent.

If you choose the simplified method, claim in Canadian or US funds a flat rate of $23/meal, to a maximum of $69/day (sales tax included) per person, without receipts. Although you do not need to keep detailed receipts for actual expenses if you choose to use this method, we may still ask you to provide some documentation to support your claim.

You have two choices: claim exactly what you spent, but you have to have receipts; or take the flat rate, but you don't need receipts. The proof mentioned for the flat rate is that you might have to have something else to justify the reason for the expense.

Because there are no receipts, with the flat rate method they have no idea if you went to the cheapest place in town and ate for less than $10, or the most expensive place in town and spent more than $23 for the meal.

In some locations $69 as day is more than most people spend, in other cities that can be difficult to keep under that number.