The encryption itself may not be.
Establishing the initial exchange of crypto key material can be.
That's where certificates are important because they add identity and prevent spoofing.
With TOFU, if the first use is on an insecure network, this exchange is jeopardized. And in this case, the encryption is not with the intended partner and thus does not need to be attacked.
But like, no: the free Wi-Fi I'm using can't, in fact, MITM the encryption used by my connection... it CAN do a bunch of other shitty things to me that undermine not only my privacy but even undermine many of the things people expect to be covered by privacy (using traffic analysis on the size, timing, or destination of the packets that I'm sending), but the encryption itself isn't subject to the failure mode of SSH.