`ec2-ssh` fails against non-EC2 managed instances

[strophy]

I have a few Raspberry Pis that are registered as managed instances mi-* in Fleet Manager. Connecting to them works fine with a command like ec2-session pi-1:

$ ec2-session pi-1
Starting session with SessionId: strophy-0f0424c62fea5e575
$ 
Exiting session with sessionId: strophy-0f0424c62fea5e575.

But ec2-ssh fails with the following behaviour:

$ ec2-ssh pi-1
INFO: Resolved instance name 'pi-1' to 'mi-048afb7f6460a110d'
ERROR: Unable to figure out the EC2 login name. Use "-l {user}" or {user}@{instance}.
$ ec2-ssh pi@pi-1
INFO: Resolved instance name 'pi-1' to 'mi-048afb7f6460a110d'
INFO: Using SSH key from SSH Agent, should be as good as any.
ERROR: An error occurred (InvalidArgsException) when calling the SendSSHPublicKey operation: 1 validation error detected: Value 'mi-048afb7f6460a110d' at 'instanceId' failed to satisfy constraint: Member must satisfy regular expression pattern: ^i-[a-f0-9]+$

Is this a restriction imposed by aws-ssm-tools or the underlying ssm start-session command? Are there any workarounds?