That's not accurateAdmittingly, I am not an expert on the Alfa's ECU but that would vary from most all other modern ECU behavior that I am familiar with.
Open loop is used for medium to hard throttle acceleration, cold idle and deceleration. Closed loop for cruising, low throttle positions and other low-load scenarios where fuel efficiency is paramount.
That's why cars run a richer AFR than stoich (which 14.7:1 on gas) when you see them on the dyno. NA cars like 12ish and boosted cars may go richer at say 11-11.5:1 They don't use the O2's to correct fueling to get back to stoich. They run open loop and look up the fueling in a table. At this point, the O2's are just reporting the news, not actually feeding info back to the ECU to adjust fueling.
This is why you have to retune for mods because 1. when running in closed loop, the short term and long term fuel trims will be out (if you are familiar with tuning software, you can view the fueling in various "cells" which show where it is lean or rich) and 2. the pre-determined tables used for wide open throttle may no longer have proper values due to changes in airflow ie. new exhaust. Additionally, for turbo cars, boost pressure can be affected by hardware changes. If the table values are too far out, a rich or lean condition can occur which the ECU wont correct for, causing issues.
Again, not saying there are ANY issues with the Fabspeed parts or anything wrong with OP's car. Just some thoughts...
The only semi modern ecu ive seen run open loop under load are some honda from 2000.
Bosch, simos, mm etc etc have been running closed loop under any load situation (only open loop during cold starts) since the early 2000s. All Audi, all BMW, all VAG, all MB, etc etc etc.
These cars run 11:1 or richer under wot simply because of enrichment tables from cat temperature correction.
They otherwise target 14.7:1 for the most part with DI for the past 15 years or so and nowadays will target even 15:1 or leaner. Thats part throttle off boost though because as soon as the cats pass various temp thresholds which they always will under WOT the ECU goes to EGT enrichment tables.
o2 sensors are providing feedback to make sure it happens.
That's why you can run a piggyback under WOT and still see 11:1 AFR. If it was open loop you'd be out of luck.
Here's the car running 5lbs more boost down low, 2 up top and no change to fueling programming but still maintains identical afr as stock curve because of closed loop. Been that way going back to bosch MED in 2000