No ClubMan, not true in my experience.
The guarentee is not underwritten by money coming out of the developers own pocket (i.e. the profit on his sales) but rather from the government, by way of a refund on the 19.6% VAT which is levied on the building costs. So there is no necessity for the developer to have to add a premium to the price to cover the rental guarentee.
Now, you might of course find some developers trying to up the price a bit because they think we're mugs, but this is where you need to do your homework and compare pricing (locally, and regionally) and be clear about value and future marketability.
In my opinion the REAL issue with Leasebacks is that (usually) the premier development sites are snapped up by straightforward developers selling to the open market with no Leaseback or guarentees. These guys know their premier location will deliver good sales without the hassle of arranging leaseback contracts, VAT refunds, and permissions with the bureaucratic French authourities.
As a result, you generally find leasebacks in what I would call 'second tier' areas or sites. Places which would be more difficult to market to a sceptical buyer without the carrot of a guarenteed income.
A few years ago - when the French property market was in the doldrums - you might find a prime located leaseback. Not any more. For instance, you will no longer find a leaseback on the Cote D'Azur sitting on a marina, or a popular beach. They are found way back in the hinterland where prices are lower.
If you bought one in the good old days, in a great location - lucky you!