Zone minutes is merely a figure like glasses of water, just something to see, not used in any calcs.
Credit is for the figure, not calories, if you did like you say, not starting a workout where HR matters.

What was the distance on each of the days you are comparing?

Because again - it's distance for calories, not steps. Steps merely gives a rough idea of how active you must be - and 33K is huge.
When you have that many steps, small per step inaccuracy can be rather big.
And differences in distance can be big.

1000 cal difference is rather big too though, but with that many steps I've seen a huge difference in distance too and resulting calorie burn.

Giving MFP only a step count and letting it figure out distance (which it doesn't show) and resulting calorie burn is even rougher estimate.
Lower doesn't mean more accurate when you only have 2 figures to compare.

During that 33K or 34K day - did you have a purposeful exercise walk/run for a big block of time? (not daily stuff interrupted with standing around)
If so, did you confirm calorie burn by creating an Activity Record (where you provide start/end time only) and seeing the stats for distance and calorie burn, and then compare that with the online formula for calorie burn.
Might be surprised.

Apple watch and Fitbit can both be set to only get steps from the device - must disconnect accounts for Fitbit to be reused that way.

Once disconnect on both sides - the accounts won't talk to each other.

Then in MFP app - you go to setting for Step Source and select Fitbit.
That means just get a step count reading from Device directly.

Fitbit does have a few models where if you start a Weight Lifting workout, it doesn't log calories per HR, but rather per database rate of burn.

They would be smart to keep using distance-based calorie burn for workouts start as Walking or Running instead of HR, or like Garmin run both calc's and weight the distance for about 80% of the calorie burn, 20% based on the HR.

For the other they are going for average usage - where these inaccuracies may cause upwards of 5% off either direction for vast majority of users.

You aren't the average user likely.

I know of 3 and seen their math that shows they are underestimated by about 15% - they'd be losing weight following the numbers for maintenance.

I was mainly having OP look at why the Fitbit was higher daily calorie burn, several things could be part of issue.
They get a lot of steps, so distance accuracy matters for the daily stuff.
The exercise is stuff that HR-based calorie burns will be inflated, always requiring a manual workout entry to get better calorie burn.
Just too many annoyances.

Apple actually reports just one daily burn figure to MFP when accounts are synced - their base or sedentary calorie burn, pretty close to MFP Sedentary burn rate.

They don't report the active calories actually.
They do send workouts.
Which makes what they send a whole lot worse with the math.