Half right. His FnF investment is mostly in common, but his amount of juniors isn't tiny.
Pershing Square's 2021 annual report shows Financial Services on page 84, with common holdings of $154,118,000 and preferred holdings of $32,791,000 (rounded to the nearest thousand), making his junior pref position 17.5% of his overall FnF position.
Relevant closing prices on December 31 2021 were $0.82 for FNMA, $0.83 for FMCC, $3.12 for FNMAS, and $2.87 for FMCKJ. Assuming that Ackman's common holdings are 64% FNMA and 36% FMCC, he has around 187M total shares between the two. This is consistent with the fact that FNMA's total shares outstanding are 64% of the FNMA and FMCC combined total (1.15B out of 1.8B) and that Ackman owns around (or just over, it appears) 10% of the common of each company.
Assuming that Ackman's junior pref holdings are 50% FNMAS and 50% FMCKJ, those closing prices mean he owned around 5.5M shares of each on December 31 2021. I haven't seen an exact breakdown of his junior pref holdings, but this is good enough for comparison purposes.
Assuming Ackman didn't sell any commons between December 31 2021 and June 30 2022, his 187M commons were worth $79,736,800 on June 30 2022 (120M shares of FNMA at $0.43 plus 67M shares of FMCC at $0.42). That puts his junior pref percentage at 31.9% on that date.
We will have to wait another 6 weeks or so to see the 2022 annual report, but if his share holdings remained the same then his junior pref percentage on December 31 2022 would be 26.9%. That's 120M FNMA at $0.35, 67M FMCC at $0.35, 5.5M FNMAS at $2.34, and 5.5M FMCC at $2.03.