Working in IT can be used as a blanket term for many fields of work, including software development as it is an information technology sub field. Just as being a programmer is technically an IT work. Help desk is IT.
“I Worked in IT for 20 years” can be a very broad statement without actually covering what the person does.
And there are companies that specifically sell software development to companies for more customized software.
Heck, I used to work for a company that used a program called cogent software, which the company I worked for has their own customized version of to allow for some interaction that other users of the same (named) program of other companies did not have. Because the company I worked for needed it to work a bit different than what the base program did.
Software companies can and do sell their product to businesses and can do custom installs with tweaks to the programs design and functions to fit the needs of the client company.