That’s a startlingly valid point!
I am left with a bit of perplexity about the purpose of the Buddha’s visit…
Sariputta clearly knows about the five higher fetters (uddhambhāgiyāni saṃyojanāni) since he discusses them in DN33. But in this sutta, he chooses to say “external fetters” (bahiddhāsaṃyojano), which would alter the meaning. Indeed, each of the higher fetters feels internal rather external. For example, it would be difficult to think of the higher fetter of conceit as “external” given that it revolves around “I am”. And although “rebirth in the realm of luminous form” would be external, the “desire for rebirth…” is internal.
Sariputta doesn’t explicitly enumerate the external fetters in the sutta, yet he does define the person restrictively:
Who is a person fettered externally?
It’s a mendicant who is ethical, restrained in the monastic code, conducting themselves well and seeking alms in suitable places. Seeing danger in the slightest fault, they keep the rules they’ve undertaken.
This would support your thesis.
However, the Buddha, doesn’t say “keep minding the rules” on his visit to Sariputta. Nor does he quibble about internal or external fetters. Instead, the Buddha says something quite tangentially odd about devas on a pin:
Those deities, though they number ten, twenty, thirty, forty, fifty, or sixty, can stand on the point of a needle without bumping up against each other.
Even living arahants can’t stand together on the point of a needle. Their bodies would get in the way. So perhaps the body is an external fetter. Indeed, everything discernible as “This is not mine, I am not this, this is not my self” would be…external to self and therefore an external fetter.
And, curiously, practicing “This is not mine, I am not this, this is not my self” does indeed lead to the peace directly mentioned in the wisdom unique to this sutta:
So you should train like this:
‘We shall have peaceful faculties and peaceful minds."
I’m still gnawing on that neutral feeling.