This is correct (well, $18,500 for 2018) but it's not the entire picture.
Under IRC §415(c) there's a $55,000 limit per employer. If you work for an airline and you're in the Guard and Reserve, you have a huge benefit here.
For example, if you're at American, the company can put your 401k defined contribution and your profit sharing in, plus any contribution you make. For 2018, the sum of all three is capped at $55,000 and your contribution is capped at $18,500 (the elective deferral limit).
Your total out-of-your-own paycheck contributions to the 401k and TSP are capped at the $18,500 elective deferral limit.* Whatever you don't contribute to the $18,500 limit at the airline you can contribute to the TSP.
*There's an additional benefit if you're deployed. Under IRC §402(g) the $18,500 limit does not apply to traditional contributions made from tax-exempt pay earned in a combat zone. If you're earning tax-free pay in a combat zone, you can contribute above the $18,500 limit up to the $55,000 limit.
If you make $150,000 at Delta in 2018 and deploy as an O-5 in the Guard for a few months, you could do this:
16% 401k $24,000
20% Profitsharing $30,000
Total Delta 401(k) $54,000
Deployed Combat Pay to Traditional TSP $55,000
Total Tax Sheltered Space $109,000.
Under USERRA, airline guys on military leave are also eligible for 401k make-up contributions for any 401k contributions you would have been entitled to while you were deployed. Generally that means whatever your pay rate is and your monthly guarantee or bid line average, multiplied by your 401k defined contribution rate.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-userra-and-sscra