I too was in "the IT game" but for over 40 years.
There is a huge - and auditable - difference between a development/support team having access to a system, and the same people being able to change data on the system.
The former is logical and beneficial, the latter is open to fraud, is...
What he said ☝️
and ... In my experience of selling and delivering key IT systems; its relatively easy for suppliers and/or "informed users" to test a system prior to implementation and find that it doesn't work to the extent that Horizon seems to have failed
and ... a basic principle of...