Since Extreme's MLAG does not interact with STP, that recommendation is sensible. The two MLAG peers use different bridge IDs when generating BPDUs.
The default settings on e.g. a Cisco IOS switch put a port-channel into err-disabled mode on receiving BPDUs with differing bridge IDs on different member ports of a port-channel. But this is only relevant if STP is enabled on the EXOS switches.
With only the C3 sending BPDUs out the LAG and the X450-G2 ignoring them, spanning tree on the C3 handles the LAG as an edge port.