MD Non-resident instructions say on page 1:
A nonresident individual is subject to tax on that portion of the federal adjusted gross income that is derived from tangible property, real or personal, permanently located in Maryland (whether received directly or from a fiduciary) and on income from a business, trade, profession or occupation carried on in Maryland and on all gambling winnings derived from Maryland sources.
That almost sounds like you would have to pay tax in MD on your MD sourced income. You would get a CA credit for the MD tax paid. Since you work remotely from CA, then the carried on in MD would not be true. Which would get you off the hook for filing MD taxes. That is a close call. Since MD sent you a letter stating that you do not have to mail, I would keep that as proof and skip the MD tax.
In 2015, Maryland got in trouble with the Supreme Court for double taxation. The states have been doing a lot of shifting how they operate since then.
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