This is a question for GOS team. But, yes, if the app is not compiled with the appropriate flags the MTE for that specific app will be bypassed, BUT..... all grapheneos is compiled with mte because the protection is inherited in the system. So if your non mte app try to corrput some component of GOS system memory which is mte enabled it will possibly be detected anyway. Memory tag is just one of the vast security procedures graphene os takes to avoid memory violation. Alone MTE is bypassable...but not in this case and as shown in the arcticle... graphene offers much more protections (hardened malloc, ASLR
for example) that togheter make the system hardened.
Maybe someone can explain better this for you.