You are asking two different questions, here.
Is it tecnically incorrect to claim that TCP/IP connection was established with an IP address?
As @terdon pointed out in a comment to the question, a TCP/IP connection is made to something that has been assigned a given IP address. If you want to be technically correct, that is very different from establishing a connection to "an IP address".
The IP address is like a phone number, whereas the device is the phone that is hooked to a phone line that has been assigned that particular phone number.
if somebody claims they established a TCP/IP connection with an IP address assigned to a cable modem would it be technically correct to tell them that this is not possible since a cable modem cannot do that?
No. What makes you think a cable modem doesn't run any software that can accept (or for that matter initiate) such a connection? A cable modem isn't just a bunch of transistors, dumb ICs, capacitors etc., it's a small computer in its own right, running software. What that software does is up to the manufacturer.
For one, my own modem (which strictly speaking is an ADSL modem and not a cable modem) can be configured in either bridging or gateway (NAT) mode, along with having a whole slew of other configuration options as well. That would be impossible if it was just a "dumb" modem that sat between two electrical interfaces and translated signals from one form to another.
There are even managed switches and (mostly if not exclusively server) motherboards out there that allow you to make IP connections to them for monitoring and configuration purposes.
You also have to consider the effects of visibility. If someone connects to an IP address and the host at that IP addresses (which may be a cable modem, home router, personal computer, printer or what have you) forwards the connection to a different IP address, to the one who connects it looks like they are communicating with the IP address they established a connection to, even if the traffic is forwarded by that host to another IP address. This is the basis for how NAT works, for example.