There are a few points to take into consideration. Firstly the VAT (that you pay for a leaseback unit) can generally be claimed back from the French authorities. This is true of all new developments but is not true for all refurbishments. Smaller refurbishments may not qualify for VAT exemption.
With LMP/LMNP status you can deduct certain expenses from your rental income for the purpose of calculating your French income tax. Examples of allowable expenses are purchase fees, loan interest, management charges, etc. You can also depreciate your unit and furniture in order to reduce your tax bill.
What your agent says may be true for the first few years i.e. deductibles (excluding loan interest) and depreciation may be sufficient to reduce your taxable income to zero (or less). However, in most cases you will find that as time passes and the rental income increases you may end up with some degree of taxable income. Therefore, the fact that you can't claim the loan interest against this income can lead to a French tax bill. You will need to work out the expected cash flows to determine the likely tax bill.
There have been many discussions about leasebacks on this forum and it is well worth having a look over the contents of the relevant threads.
Regards,
Paidi