You should probably find a new lender. The interest you pay into your 401(k) is not taxed as income.
I think what he means is that the interest that you pay into a 401(k) loan is after tax dollars on your paycheck, so it has already been taxed once, and then, when you withdraw it after retirement, it is taxed again as income.
An example (and I hope I get it right this time):
You borrow $10,000 from your Standard (not Roth) 401(k), spend it on new avionics, and pay the loan back with your next paycheck (interest of $1.64)
(because you're making $166,625K/year and you live in WA, FL, TX, or some other state with no income tax):
Your paycheck (use the monthly column, this is from
http://us.icalculator.info ):
Gross Pay: $ 13885.42
Exemptions: $ 858.33
Taxable Income: $13,027.08
Federal Inc. Tax: $ 3070.19
FICA $ 612.25
Medicare: $ 201.34
Net Pay: $ 10,001.64 (Used to pay back 401(k) loan. You earned $13,885.42 to pay back $10,001.64.
So, you earned $13,885.42 (gross pay), but only put $10,001.64 (taking the take home pay and paying back the loan, so your check will be $0) back into your 401(k).
That loan cost you $3883.78 in taxes when you repaid it. (For those of us that don't make $166,625, that number will be less, but hey, you're on an aviation website, so it won't be much less.)
The next month, you retire, and take $10,001.64 (gross, the amount you just paid back) out of your 401(k) (yes, you're over 59 1/2):
Gross Pay: $10,001.64 (your monthly gross from the 401(k)
Exemptions: $ 858.33
Fed. Inc. Tax: $ 1,982.73
FICA $ 612.25
Medicare: $ 145.02
Take Home Pay $ 7,261.64 Taking out the amount you just paid back nets you about $2800 less, due to the taxes paid on it as ordinary income.
That's why I advise against it. Your repayment and interest is double taxed. It gets worse in states that tax income as well. When looking at these numbers, I'm not sure what a "circumstance exemption" is, but the point I'm making is that your repayment and your interest paid on the loan is after tax money at repayment, and taxed again as regular income at retirement withdrawal.