I can not say that it only defaults to GPT.
On the contrary: It doesn't give any alternative to GPT at all.
I've tested this extensively on several laptops with legacy BIOS:
It doesn't fall back to MBR nor gives it a choice to do so.
It creates GPT wether you like it or not.
With legacy bios it creates a fake partition to make room for a GRUB extention to make the legacy BIOS capable of working with GPT at all.
See this:
To quote the relevant part:
After install on my ASUS F3T, which has a BIOS dated 2005, which predates both GPT and UEFI by several years, I ended up with:.
a GPT partitiontable (!?!), 1 GRUB partition, 1 EFI partition(!?!) and 1 EXT4 partition.
And it just ... works.
But a Sony VAIO of that age doesn't because of BIOS incompatibilities
Also a Toshiba P100 I have couldn't because of BIOS incompatibilities
My T420 won't boot the installer in legacy mode (it does in UEFI mode)