Well, what I'm about to write is just the fruit of my hypotheses. It seems that Spider-Man was supposed to appear in Madam Web. This is also supported by concept art that appeared on the internet.How official is the word on "Tom Holland can't be in a Sony movie"...is it just a rumor/assumption, or has it been stated as a fact by those with the authority to do so?

Tom Holland's Spider-Man Goes Toe-To-Toe With Ezekiel Sims in 'Madame Web' Concept Art
New Madame Web concept art shows an MCU collision with Sony's latest spider-movie. See Tom Holland's Spider-Man fight Ezekiel Sims in the new images.

What some people assume is that the film began production with a script that included Spider-Man (in fact, Dakota Johnson has complained several times that the script she started with was very different from the final one). Also, there was a rumor that the actress' agent had told her that she would be working in a Marvel film (in the MCU sense). In fact, the first announcement about the film on Twitter also contained the Tag #marvel
For some reason, his appearance couldn't happen. But they found themselves with a good part of the film already shot. So they had to adapt the film during production and retroactively change the plot, even with a heavy use of ADR (the excessive use of this is something many have complained about).