Purchasing I Bonds with tax refund

The website for Treasury Direct, the government site for electronic purchase of U.S. savings bonds, etc., states that $5,000 in I-Bonds can be purchased with tax refunds FOR EACH SOCIAL SECURITY NUMBER of the taxpayer(s) on the return. Individual - Comparison of TIPS and Series I Savings Bonds (treasurydirect.gov)   

(See "Purchase Limits")

 

In other words, a single filer can purchase a maximum of $5,000 in I-Bonds with a tax refund by including Form 8888 with the return, and $5,000 in I-Bonds can be purchased for EACH taxpayer filing jointly, for a maximum of $10,000.  However, the TurboTax IRS Form 8888 will not allow more than $5,000 maximum purchases of I-Bonds with tax refunds, even if the return is a joint one.

 

Is the Treasury's website explanation incorrect - meaning the maximum purchases of I-Bonds per return is $5,000, whether the return is for a single filer or a joint return, or is Form 8888 not correctly permitting at present a maximum per joint return of $10,000 of I-Bond purchases?