As a triathlete you don’t practice butterfly to get good at butterfly (although it doesn’t hurt). You practice and do butterfly to improve your freestyle. Doing butterfly is the most effective strength work you’ll do in the water. It’s better than paddles work (paddles increase strength endurance mostly, not raw strength), and better than any unilateral movement (freestyle is unilateral) to increase strength.
After the butterfly is a pretty challenging hypoxic set. Remember, on hypoxic sets if you have a very weak side that you avoid breathing on, you can breathe exclusively to that side and breathe one stroke more often than the workout prescribes.
The main set today has some actual butterfly in it and it’s challenging. The idea behind the main set is once you miss the interval doing the prescribed butterfly, you’ll switch over to freestyle and stay on the same intervals. It will be tough, but not impossible. It is a threshold workout after all.