The answer to your question depends on your risk tolerance. Only you (and spouse/partner) can determine that. Paying off or prepaying the mortgage is zero risk with a guaranteed rate of return of Mortgage rate * (1-Marginal tax rate), assuming you exceed the standard deduction; otherwise it's the mortgage rate.
Do you hold bonds or bond funds in any of your accounts? If so, you can substitute bond holdings with prepaying mortgage (3.75% is way better than what decent grade bonds are paying...).
Prepaying mortgage IS savings.
Re: the statement in bold above - not necessarily. In fact, prepaying can actually cost you money in the long run.
Assessing risk tolerance should be done after studying the topic thoroughly, not just relying on "feelings."
The links helpfully provided by nereo are an excellent place to start.
But the statement is right.
Keeping 80% of your net worth in a savings account is "savings" - it's just savings that's going to lose its value over time due to inflation, and give you a very poor return compared to alternatives, but it's approximately guaranteed not to vary in value.
I don't disagree with your bigger point, of course: Excessive amounts of your portfolio in too-conservative savings has substantial inflation risk, and (counterintuitively) actually increases the risk of your portfolio not making it through your target.
I didn't say it was wrong, I said "not necessarily". This site is dedicated to reaching FIRE in the most efficient way possible. Pre-paying cheap, affordable, fixed-rate mortgages just ain't it, for most people. The two significant exceptions, IMO, are folks who live in countries there mortgage interest is not tax deductible (Ouch!), and folks who live in flyover country, where perfectly suitable housing can be procured for well below $100k.
Otherwise, I think we are singing in the same choir, csprof.