That is generally true.

I see you want to cut the treads into a pie shape. We really need more information.

Like, are these treads for a circular stairway? Or a several step landing?

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You're not going to find a 16" wide piece of solid wood, unless you want to spend a fortune on it. Nothing wrong with gluing, properly done the glue joint is stronger than the wood itself.

It's a spiral staircase wrought iron pole and 12 steps.
Wanted solid oak wood but can't buy anyway. Husband is handy and can cut out pie steps.

There will be a mathmatical formula for the shape. I do well to add/subtract, so find a math teacher/architect/etc. for the figuring.

Glued together solid wood is still solid wood.

Buying 16" wide oak is not difficult. I think you misunderstood if a"lumber mill"is indeed where you asked.
Oak 24" wide is cut daily by "lumber mills". The boards are ripped into narrorer boards after leaving the mill and before sold because a wide board will cup whereas wide glue ups will not,or at least not near as much. Have your husband discuss with mill what you need and they will instruct him on how to rip and glue. They will also talk about plain sawn,riff sawn,quarter sawn and other options that will hide glue lines to appear as a single piece.