Man here! (Sorry, I really try not to do that but it's relevant here)
I don't really see why your husband's story is that unlikely @Sunsh89 . I can certainly think of times I've sent messages, or done random bits of life admin, while also masturbating.
I think men have two "modes" of masturbation, while women don't. Just going off women I've been out with in the past, they tended to set time aside specifically for the purpose of masturbation. And I do that sometimes as well. It's "I am setting aside this time specifically with the aim of orgasming"
But men also do what'l I'll term the "boredom wank". Where I'm not specifically looking to have a wank, but maybe this TV show isn't really gripping me, or maybe I'm scrolling Reddit and one in every 4 links I click on is porn. And if I orgasm, I orgasm, and if I don't , I don't. I'm not really masturbating, I'm just playing with my dick.
And I don't think women really do the second one, because I think generally masturbation involves more concentration for women than men. Ironically, given the male sexes reputation for being unable to multitask, this is the one thing we really can multitask during.
So yeah, I've done other stuff whilst masturbating, whether it's getting up to get a chocolate bar, or messaging DP to say we're out of chocolate bars, or setting a reminder to pay the gas bill, or checking IMDb to figure out who the actor on the TV show I'm watching is. Because I'm not sat there fantasizing at the time, I'm just doing something to occupy my hands that also feels good.
I'm not saying your husband definitely isn't cheating OP, just that what you've got isn't rock solid evidence by itself.
There's also the chance he's using one of those AI apps, to play out some fantasy. They send messages as well as do image generation,which would explain the mix of messages and porn.