Peter Ustinov's humor: I don't get this one

Factual Questions
1

A few days go I saw a rerun of his appearance on Cavett. But I didn’t get the a bit of his humor that Cavett did.

Ustinov was speaking about his great-uncle, saying he “was one of the founders of Russian ballet: he did the sets of Petroushka, worked with Stravinsky”. The uncle once invited to spend a weekend at Tolstoy’s estate, but became worried that Tolstoy would “dominate” him. So he sent a telegram saying he was ill. Ustinov asked him if he’d ever regretted sending that telegram. His uncle thought about it, then replied, “No…”

Ustinov: He was still thinking, “I was right, 'cause the weekend would’ve been a bore. He would’ve spouted bits of ‘Anna Karenina’ at me and it would’ve been a nuisance.”

Cavett: Yes, we know how Tolstoy would bore people.

Ustinov: That’s right. One Tolstoy after another.

[after 1 second, Cavett groans loudly and laughs, and there are some groans and laughter from audience]

Cavett: I find it in my heart to forgive you for that.

Ustinov: Thank you, my son.

Can you give your opinion without reading the spoiler here: is the reaction for a pun on “Tolstoy”/“tall story”? To me that doesn’t seem to merit Cavett’s reaction, so perhaps I’ve missed something.

2

That’s how I’d read it: it’s a pun.

3

I think it’s a sort of pun on “one tall story after another”.

4

It’s a pun on tall story, yes.

People used to think a good spontaneous pun was the sign of a quick wit, and not just a Dad joke.

5

I imagine he pronounced it the Russian way, with the emphasis on the second syllable, which makes the pun a bit less forced.

6

I rewatched it and the syllables had about the same emphasis. But I’m glad everyone confirms my interpretation. Thanks to all.

7

It could have been a pun on dull story rather than tall story.

8

As a pun, Ustinov’s effort is like the call of a guinea pig.

Weeick.