Senior transit in Warsaw

I think you do need to be a Polish citizen to get discounts but I am not sure (can you provide any links as to the source of your info? how would you get a card verifying your eligibility?). Whatever the case, the transit is so inexpensive (especially with a $USD to zloty exchange rate) that I would not worry about it. It will not break the bank.

http://www.ztm.waw.pl/?c=142&l=2

Long list of reduced, includes:
"People over 70
ID card or another document with a photograph showing date of birth"
No mention of citizenship.
Yes I know it is cheap but free is more convenient.

That's definitely an official site and indeed it says that folks over 70 need to show ID with birthdate. So you are right!

Also noticed in: https://www.inyourpocket.com/warsaw/public-transport_21173v

A standard public transport single ticket costs 4.40zł. If you’re travelling to the further reaches of Warsaw you’ll be needing a ticket that covers both zones 1 and 2 – these are priced at 7zł. Note that the airport is in Zone 1. Still with us? Good. There is also a 20 minute ticket priced at 3.40zł. Tickets valid for 24 hrs are priced at 13 or 26zł if travelling through both zones. They have also introduced a new weekend ticket (available from 19:00 on Friday till 08:00 on Monday) which costs 24 zł (also a weekend group ticket is available for up to 5 people and costs 40 zł). Those over 70 ride for free, as do children up until the end of September of the year they turn 7.

http://www.ztm.waw.pl/informacje.php?i=555&c=98&l=2

Discounts in public transportation for foreigner visitors

Please note that also foreigners can use public transportation for free or at a discount.

Foreigner visitors who meet the criteria set out in the transport tariff of the Municipal Transport Board can benefit from free or discounted public transportation in Warsaw.

Individuals entitled to free transportation:
- Children, from their birth date to the 31st August of the calendar year in which they turn 7,
- Persons who are 70 years old or more - based on the identity card or other document with a photo that states their birth date,

Holders of the ISIC card, valid for the calendar year, are entitled to the 50% discounted transportation fee, provided they are students of a foreign higher or secondary school, and are no more than 26. In addition, a foreigner who is 65 or more, can purchase a Senior Ticket - the ticket is valid with an identity document confirming the age of the passenger.