How to deal with this rejection email from the journal Mathematics of Computation?

It is just a nice way to say no. Submit elsewhere.

This is called a desk reject; you will find more in this question What does the typical workflow of a journal look like? .

Note that Math Comp is one of the top journals in its field, so unless the paper was extremely good this was the expected outcome.

It's totally normal to get a first paper rejected, especially if you are dealing with a top notch journal (receiving lots of submissions) in a time of reviewer shortages. Don't take it personally or feel bad about it.

I would encourage you to submit your article to another journal. You can research potential journals by looking at what journals your citations were published in as well as looking at lists of journal rankings (e.g. SJR, list of these ranking websites). Note: don't get too hung up on exact rankings - think of these lists more as a tool where you can search journals by keyword and try to get an understanding of who has a history of being being a reputable journal versus who is a potential scam. Come up with a list of a few suggestions, and then go talk your ideas over with your supervisor. They may have other suggestions too, but it's good to start learning about how to pick a journal.

Even if the backlog answer was true, it could take many months to clear it out. I would submit to another journal.