Arecoline, the main alkaloid of betel nut has a pKa of 7.7. So at pH 7.7 50% is freebased. At pH 8.7, 90% is freebased, and at pH 9.7 99% is freebased.

The mouth has a pH of about 7.0. It varies though. At pH 7.0 less than 50% of the arecoline is freebased so it's poorly absorbed.

Freebased drugs like freebase arecoline absorb through the mouth more quickly than salt forms do. So to make it absorb better, you need to bring the pH of the mouth up.

Sodium bicarbonate is too water soluble. It immediately dissolves and is quickly neutralized by the acids in the mouth. So it only brings the mouth to pH 8.3 for a very short time. Because of that, it’s one of the least effective bases for oral use or intranasal use.

Like calcium hydroxide, calcium carbonate is almost insoluble in water so it doesn’t rapidly dissolve and stays there in your mouth keeping the pH at 9.8 for a long time allowing the arecoline more time to absorb as freebase. So it should be far more effective than sodium bicarbonate.

These same rules also apply to snuff like Yopo.