well sometimes it is true, for example older steel bike from the 90s will maybe just as strong as some these days because they were build to be robust, where as these days they TEND to be built for lightness
Most earlly steel and titanium hard tails were lighter than the modern equivalents, as were some of the full sus bikes. My original Mount Vision is still doing the rounds with its second owner and it was lighter than my current Marin. My Trek 930 has outlasted a few steel bikes, and has been in constant use rain or shine on road and off since the mid 90's.
I personally feel something may be not going well with Commencals choice of alloys, welding techniques and geometry, considering the weight and seat angle, the tubing thickness and material, I am not too surprised by the seat tube-seat post mast problem. Many "other" manufacturers suffered such failures during developement, and due to this many "other" manufacturers use additional structure support with that design criterea. I was not too fond of thier engineering principles a couple of years back, where one of thier designs clearly was not put together with the chain length and triple ring spec in mind. And a good look at many of thier framesets does not inspire.
The bikes may ride well, but in my honest opinion, the build quality and attention to detail are NOT what you are paying for