Ottelenghi is indeed not known to me. Unsurprisingly, given that he is a vegetarian chef (of course, I looked him up), his and my meat-focused world do not overlap. At all!

With respect to the bioavailability of polyphenols and the effect of banana PPO, the subject needs to be put in perspective.

First, the bioavailability of polyphenols is generally very poor. That is, the proportion of intact polyphenol molecules that make it through the digestive tract and into the blood is in the range of less than 1% to 10% max, depending on the individual compound. This suggests that those polyphenols are metabolized into other compounds that exert their beneficial health effects through other pathways. One of those pathways is a favorable composition of the microbiome. So, when studies investigate the bananas' PPO effect on the bioavailability of intact polyphenols, we probably miss an important part of the story.

Second, the dramatic drop in bioavailability in those studies is only seen when the banana-containing smoothie isn't consumed immediately after preparation. Banana PPO reduces polyphenols already before ingestion, when you let the smoothie sit for a while.

When the smoothie is consumed directly after preparation, the bioavalabilty is only reduced by roughly 10-15%.

Adding lemon juice as a concentrated form of ascorbic acid (a known PPO enzyme inhibitor) reduces the PPO effect. Whether that makes sense in your combination with oranges, is questionable because the latter already deliver a high dose of ascorbic acid. But then again, adding that squeeze of lemon can do no harm